[macOS] Use native window drag for the custom editor title bars.

[bruvzg]

Fixes godotengine/godot-proposals#11214

• Adds DisplayServer::window_start_drag function to start window drag as if title bar was clicked (drag process is controlled by the window manager, and allow window to use tiling).
• Changes EditorTitleBar to use these window drag mode when available.

[Riteo]

Thank you, this API point will be extremely useful for the wayland backend too, as it's the only way of moving a window there :D

[bruvzg]

this API point will be extremely useful for the wayland backend too, as it's the only way of moving a window there :D

In this case, I'll update PR to use a separate feature flag, instead of binding it to existing FEATURE_EXTEND_TO_TITLE.