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)

Edited by Simon Ser

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