xdg-shell: introduce capabilities

Some compositors don't implement all of the features of xdg-shell. This results in UI elements (e.g. buttons) in clients which do nothing when activated.

Add a capabilities event to allow clients to hide these UI elements when they don't make sense.

Signed-off-by: Simon Ser contact@emersion.fr Closes: #64 (closed)

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• Review: @jadahl
• 3 implementations:
  • foot: https://codeberg.org/dnkl/foot/pulls/1061
  • wlroots: wlroots/wlroots!3543 (merged)
  • Jay: https://github.com/mahkoh/jay/pull/39
  • weston: weston!933 (merged)
• 3 ACKS
  • @jadahl for Mutter
  • @daniels for Weston
  • @zzag for KWin (on IRC)