Port to new Clutter/Wayland API

Signed-off-by: Daniel Stone <daniel@fooishbar.org>
Default theme: Add window close icon from GTK+

Signed-off-by: Daniel Stone <daniel@fooishbar.org>
Remove padding specifiers for close button

All these seemed to do was to make it expand on hover, and be
unclickably small otherwise.

Signed-off-by: Daniel Stone <daniel@fooishbar.org>