Implement system-provided file selection display on iOS

[frenzibyte]

• Closes Browsing files on iOS should use the system file chooser dialog osu#20581
• Closes Can't select song/picture for create new map on iPadOS osu#25966
• Closes Can't add audio in editor on iOS/iPadOS devices osu#30395

I'll preface this with a warning that I'm not quite experienced in the world of UIKit, and the implementation in this PR came from a one/two-days of researching Apple's documentations. I'm still unsure about the way I'm disposing the view controller on Dispose but I don't sense any issues on real-world testing.

This is implemented in a manner where if a FileSelector is loaded, it checks whether the platform supports system-based file selection, and hides the contents of the FileSelector in favour of that.

This is accompanied with osu!-side diff for the final behaviour shown in the video below.

osu!-side diff

diff --git a/osu.Game/Graphics/UserInterfaceV2/FormFileSelector.cs b/osu.Game/Graphics/UserInterfaceV2/FormFileSelector.cs
index 81023417a5..924a2dabfe 100644
--- a/osu.Game/Graphics/UserInterfaceV2/FormFileSelector.cs
+++ b/osu.Game/Graphics/UserInterfaceV2/FormFileSelector.cs
@@ -255,6 +255,8 @@ private partial class FileChooserPopover : OsuPopover
             protected override string PopInSampleName => "UI/overlay-big-pop-in";
             protected override string PopOutSampleName => "UI/overlay-big-pop-out";
 
+            private readonly FileSelector fileSelector;
+
             public FileChooserPopover(string[] handledExtensions, Bindable<FileInfo?> currentFile, string? chooserPath)
                 : base(false)
             {
@@ -264,7 +266,7 @@ public FileChooserPopover(string[] handledExtensions, Bindable<FileInfo?> curren
                     // simplest solution to avoid underlying text to bleed through the bottom border
                     // https://github.com/ppy/osu/pull/30005#issuecomment-2378884430
                     Padding = new MarginPadding { Bottom = 1 },
-                    Child = new OsuFileSelector(chooserPath, handledExtensions)
+                    Child = fileSelector = new FileSelector(chooserPath, handledExtensions)
                     {
                         RelativeSizeAxes = Axes.Both,
                         CurrentFile = { BindTarget = currentFile }
@@ -273,24 +275,43 @@ public FileChooserPopover(string[] handledExtensions, Bindable<FileInfo?> curren
             }
 
             [BackgroundDependencyLoader]
-            private void load(OverlayColourProvider colourProvider)
+            private void load()
             {
-                Add(new Container
+                if (fileSelector.UsingSystemFileSelector)
                 {
-                    RelativeSizeAxes = Axes.Both,
-                    Masking = true,
-                    BorderThickness = 2,
-                    CornerRadius = 10,
-                    BorderColour = colourProvider.Highlight1,
-                    Children = new Drawable[]
+                    // keep present for FileSelector scheduling to still work.
+                    Body.AlwaysPresent = true;
+                    Body.Hide();
+                }
+            }
+
+            private partial class FileSelector : OsuFileSelector
+            {
+                public FileSelector(string? initialPath = null, string[]? validFileExtensions = null)
+                    : base(initialPath, validFileExtensions)
+                {
+                }
+
+                [BackgroundDependencyLoader]
+                private void load(OverlayColourProvider colourProvider)
+                {
+                    TopLevelContent.Add(new Container
                     {
-                        new Box
+                        RelativeSizeAxes = Axes.Both,
+                        Masking = true,
+                        BorderThickness = 2,
+                        CornerRadius = 10,
+                        BorderColour = colourProvider.Highlight1,
+                        Children = new Drawable[]
                         {
-                            Colour = Color4.Transparent,
-                            RelativeSizeAxes = Axes.Both,
-                        },
-                    }
-                });
+                            new Box
+                            {
+                                Colour = Color4.Transparent,
+                                RelativeSizeAxes = Axes.Both,
+                            },
+                        }
+                    });
+                }
             }
         }
     }
diff --git a/osu.Game/Graphics/UserInterfaceV2/OsuFileSelector.cs b/osu.Game/Graphics/UserInterfaceV2/OsuFileSelector.cs
index addea5c4a9..8c6477090b 100644
--- a/osu.Game/Graphics/UserInterfaceV2/OsuFileSelector.cs
+++ b/osu.Game/Graphics/UserInterfaceV2/OsuFileSelector.cs
@@ -29,7 +29,7 @@ public OsuFileSelector(string? initialPath = null, string[]? validFileExtensions
         [BackgroundDependencyLoader]
         private void load(OverlayColourProvider colourProvider)
         {
-            AddInternal(new Box
+            TopLevelContent.Add(new Box
             {
                 RelativeSizeAxes = Axes.Both,
                 Colour = colourProvider.Background5,

---

Preview:

ScreenRecording_12-07-2024.04-17-13_1.mov

[peppy]

@frenzibyte conflicts 🙏

[frenzibyte]

Conflicts should be resolved. Also added minor iOS window API to do away with some of the awkward hookings and the sprinkled null forgiving operators everywhere in the code.