xdg-shell: Add suspended toplevel state

Add a toplevel state to indicate that surface repaints have been suspended. This may arise due to occlusion, output power state, etc.

In this state, clients can choose to take meaningful action such as suspending any processing which would drive a repaint loop, or communicating to the active browser tab that the tab is not system-visible, or any other action that would be taken by a client not expecting to repaint until further notice.

cf. discussion in !99 (closed)

• Compositor implementations:
  • Mutter: https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3019
• Client implementations:
  • GTK4: https://gitlab.gnome.org/GNOME/gtk/-/merge_requests/6013
  • Qt: https://codereview.qt-project.org/c/qt/qtwayland/+/485511/1
  • mpv: https://github.com/mpv-player/mpv/pull/11715
  • SDL: https://github.com/libsdl-org/SDL/pull/7846
  • libdecor: libdecor/libdecor!126 (merged)

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cc @JoshuaAshton @swick @daenzer @riteo