Disable close, move, resize, maximize and minimize functions for parent of modal window on Linux.

[EgorRudakov2]

What does the pull request do?

On Linux, disables close, move, resize, maximize and minimize functions for parent window while it has a child modal window.

What is the current behavior?

Parent window (passed as an 'owner' parameter to the Window.ShowDialog method):

• On Windows, user cannot close, move or resize window while modal child window is open.
• On Linux, window can be closed, moved, or resized while the modal child window is open.

What is the updated/expected behavior with this PR?

Same as Windows: parent window cannot be closed, moved or resized while modal child window is open.

How was the solution implemented (if it's not obvious)?

If X11Window object is marked as disabled, it ignores WM_DELETE_WINDOW event (to prevent window manager closing the window), and also changes _MOTIF_WM_HINTS to mark the window as not resizable (and not maximizable/minimizable).
Unfortunately, changing _MOTIF_WM_HINTS also removes maximize/minimize buttons from window menu bar.

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

Obsoletions / Deprecations

Fixed issues

Partially fixes #2958

[EgorRudakov2]

@dotnet-policy-service agree

[maxkatz6]

I can't really review linux specific PRs with confidence, but is it safe to disable window closing for Windows with IsEnabled=false? It will affect more than just dialog owners. Should be child window closed with the owner together?
cc @kekekeks @jmacato

[EgorRudakov2]

This code only affects events from the OS window manager and prevents users from closing disabled windows (e.g. owner of the modal window). You can still close a disabled window via API (Window.Close).