Add exclusive window handling to DisplayServer (on macOS and Windows).

[bruvzg]

I'm not entirely sure if this is the intended way to handle it.

I assume exclusive window should stay on top of it's parent and prevent focusing of the parent, and transient just mean it won't exist longer than a parent.

There's a limitation with macOS windowing system which (in some configs) prevents "child window" (which always stay on top of the parent) to be on the other screen (or virtual desktop) than its parent window.
Which should be OK for the exclusive dialogs (like file open), but is not OK for transient windows (e.g., undocked property inspector, etc.).

This PR adds function to set exclusive mode to the DisplayServer, changes behavior on Windows to keep only exclusive children windows on top of the parent (instead of all transient), and adds the same functionality to the macOS.

Fixes #57152
Fixes #57831

[bruvzg]

With this, window types should have the same (as much as it's possible) set of traits on Windows and macOS.

• Normal window:

  • can become full-screen window.
  • can not be displayed on top of another non-exclusive full-screen window.

• Transient window:

  • can be displayed on top of non-exclusive full-screen parent window.
  • can't become full-screen window itself.
  • if it has focus, returns it to the parent window when it's closed.

• Exclusive window (should be also transient and have same properties):

  • always on top of its parent.
  • blocks input to the parent.
  • on macOS (because of system limitations) it's limited to the same screen as parent and moved when the parent is moved.

[mhilbrunner]

Looks good, and I can confirm this works and fixes the linked issues on Windows.

[akien-mga]

Thanks!