Add Window scene, openWindow action, dismissWindow action, fix window close handling, and add multi-window support to WindowGroup

[kaascevich]

This PR:

• Fixes handling of closed windows (fixes Properly handle closed windows #249)
  • Scene graph nodes will now drop their window reference(s) when a window is closed, allowing the window object to deallocate properly (fixes (AppKit backend) Closed windows still show up in Stage Manager #280)
  • The backend methods used for this, close(window:) and setCloseHandler(ofWindow:to:), have been implemented in all supported backends as well as in DummyBackend
    • Currently these methods do nothing in UIKitBackend; we may implement them properly in the future for Mac Catalyst and visionOS (and perhaps iPadOS, if it'll let us)
• Adds a Window scene and its associated WindowNode
  • Only one instance of Window can be open at a time
  • Window requires a title and an ID
• Implement multi-window support for WindowGroup
• Implements the dismissWindow environment action, which closes the enclosing window
• Implements the openWindow environment action, which opens a Window or WindowGroup given its ID (partially deals with Add a way for window groups to be reopened #250)
• Factors out the window setup and view graph updating code into a new (internal) WindowReference type
• Adds the defaultLaunchBehavior(_:) scene modifier, which controls whether scenes appear on app launch
  • The automatic option matches the current behavior, namely that WindowGroups are opened on app launch but Windows aren't
• Adds the supportsMultipleWindows environment value, which can be used to check whether the current platform supports multi-window
  • As far as the SCUI-side implementation is concerned, they all do, but practically speaking mobile backends don't typically support it

[stackotter]

While reviewing #299, I realised that closing windows will need special consideration in WinUIBackend. Currently, WinUIBackend stores a list of all open windows, for the purpose of selecting one of them to attach to when modals are presented without a specific window.

You'll either need to update that list when windows get closed, or update WinUIBackend's various modal implementations to get a default window to use in some other way.

[kaascevich]

I'll take the "update the window list" route, thanks for the catch!

[kaascevich]

While reviewing #299, I realised that closing windows will need special consideration in WinUIBackend. Currently, WinUIBackend stores a list of all open windows, for the purpose of selecting one of them to attach to when modals are presented without a specific window.

You'll either need to update that list when windows get closed, or update WinUIBackend's various modal implementations to get a default window to use in some other way.

It looks like you're way ahead of yourself here:

window.closed.addHandler { _, _ in
    self.windows.removeAll { other in
        window === other
    }
}