staging/dialog: Add "dialog" protocol

This simple protocol definition allows clients to express a "dialog" relationship of a toplevel with its parent and extend the possible hints. This allows compositors to attach certain behavior according to these hints.

Requirements for merging

• Review (@swick)
• Implementations
  • https://invent.kde.org/plasma/kwin/-/merge_requests/4244
  • https://codereview.qt-project.org/c/qt/qtwayland/+/490397
  • https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3557
  • https://gitlab.gnome.org/GNOME/gtk/-/merge_requests/6837
• 3 ACKs
  • KDE (@zzag)
  • Qt (@eskilblomfeldt)
  • GNOME (@swick @carlosg)