staging/wm-gestures: Add WM "gestures" protocol

This is a high-level protocol to let compositors attach window management logic to certain gestures or actions happening on a toplevel.

Requirements for merging

• Review
• Implementations
  • https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3558
  • https://gitlab.gnome.org/GNOME/gtk/-/merge_requests/6838
• ACKs from members