xdg-shell: introduce toplevel wm_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 wm_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