Add SwiftUI proof of concept

[zntfdr]

Partially solves #281.

This PR adds a new example app which mimics the Maps.app, written in SwiftUI.

The code works for iOS 13+, however:

• the project has been created with Xcode 13
• the project uses the SwiftUI lifecycle (iOS 14+)

If it's ok, I will leave this PR as a draft until Xcode 13 is released.

In the meanwhile, I'm happy to receive any feedback you might have 😃

[zntfdr]

Thank you for the kind words!

View+floatingPanel.swift and FloatingPanelView.swift are not ready to be embedded into the main library yet, there are a couple of challenges that need to be addressed first:

1. FloatingPanelControllerDelegate (currently hard-coded here https://github.com/scenee/FloatingPanel/pull/481/files#diff-2506b1aa7d7b41f96a92bc3b56ca1ebea04e5ddc6cd8401490016f1511e9b4e8R62)
2. SurfaceAppearance (currently hard-coded here https://github.com/scenee/FloatingPanel/pull/481/files#diff-2506b1aa7d7b41f96a92bc3b56ca1ebea04e5ddc6cd8401490016f1511e9b4e8R72)

I will see what I can do to make the implementation more generic :)

[scenee]

Thank you, @zntfdr. There is no problem to do that even after this PR. I'm looking forward to your improvements step by step. 😉

[zntfdr]

Thank you @scenee, I'm currently testing a generic implementation with my app (which uses 4 floating panels), and it seems to work great 😃

Once I complete the migration and documentation I will bring everything back in this PR 🙌🏻

[zntfdr]

Update: the implementation now is fully generic!

[scenee]

I just took a look at the latest version. It looks so amazing!! I'm going to read your PR more closely later 😄

[scenee]

I found an issue that the surface view's layoutSubviews() isn't called around tip state. It looks SwiftUI issue because it doesn't occurs when the absolute inset of tip state is greater than 90. I think this issue doesn't block your PR and it's better to release this as beta version and blush up the implementation step by step.

[zntfdr]

Thank you @scenee!
Now that we have Xcode 13 RC out, I think it's ok to merge 🙏🏻

[scenee]

@zntfdr Can I move your SwiftUI API code into the library?

[scenee]

@zntfdr told me an issue on SwiftUI's environment propagation into FloatingPanel.

SwiftUI's environment is propagated to all subviews. However Floatingpanel is not a subview, but a new view controller in the screen (and not a child view controller).

I believe people using floating panel would expect the environment to be propagated like other views, but this doesn't seem to be (always) the case.

It's possible to lead behaviors unexpected by SwiftUI users so that I will merge this PR as a sample code.

[skywalkerlw]

have we fully supported SwiftUI now, or it is still just POC?

[scenee]

SwfitUI APIs are still PoC because we acknowledge there are some issues depending on SwiftUI.