This PR adds view modifiers for controlling properties of the attached view's enclosing window. Currently, this includes:

• windowDismissBehavior(_:)
  • Sets whether the window can be closed by the user
    • This will (ideally) not affect dismissWindow when that's merged in
  • Implemented for AppKitBackend, GtkBackend/Gtk3Backend (broken on macOS at least), and UIKitBackend for iOS/tvOS 16+
  • Implementation for WinUIBackend is possible, but AFAIK requires dropping down to Win32
• preferredWindowMinimizeBehavior(_:)
  • Sets whether the window can be minimized by the user
  • Implemented for AppKitBackend, UIKitBackend for iOS/tvOS 16+, and WinUIBackend (untested)
  • Implementation for GtkBackend and Gtk3Backend doesn't look like it'll be possible without an entirely custom title bar (and the minimize button is usually hidden on GNOME anyway)
• windowResizeBehavior(_:)
  • Sets whether the window can be resized by the user
  • Behaves identically to how windowResizability(_:) did before this PR
    • .automatic and .enabled behave like .contentMinSize, .disabled behaves like .contentSize
  • Implemented for all backends except UIKitBackend

They're view modifiers rather than scene modifiers because that's what SwiftUI does.

In addition, windowResizability(_:) has been updated to match SwiftUI's behavior (which resolves #263). .contentMinSize limits the window's minimum size but not its maximum size, while .contentSize limits the minimum and maximum sizes. (An extra computeLayout call is made to the view graph to compute the maximum size, but only when it's actually needed.) Maximum size limits are:

• implemented in AppKitBackend and UIKitBackend
• currently unimplemented in WinUIBackend (as are minimum limits), but probably possible with some Win32 trickery
• not implemented in Gtk[3]Backend, and probably never will be