feat(windows): Added event to forward TitleBar HwndProc to avoid adding additional hooks and timing issues

[Difegue]

Customizing titlebar behavior has proven difficult ever since WPFUI switched to using a Hwnd Hook to process WM_xxx messages.
Notably, customizing the drag regions requires either XAML hacks or additional hooks on top of the base WPFUI one.

This change allows client apps to use and override HwndSourceHook if desired, to either add additional functionality or block the base one.

Pull request type

Please check the type of change your PR introduces:

• Update
• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes

What is the current behavior?

HwndSourceHook cannot be interacted with in any way when using the Titlebar control

Issue Number: N/A

What is the new behavior?

• An additional WndProcInvoked event has been added that can be subscribed to from the Titlebar
• This event is called with HwndProcEventArgs containing the hwnd and the hook when the titlebar receives a WM_xxxx message
• Client apps can use e.Handled to bypass WPFUI's handling of said messages

Other information