xwayland: add optional support for libdecor

When running rootful, the Xwayland window is not decorated (as all
Wayland surfaces), which makes it quite inconvenient to move on screen.

libdecor is "a client-side decorations library for Wayland clients"
which can be used precisely for adding decorations to Wayland surfaces.

Add optional support for libdecor in Xwayland to gain decorations when
running rootful and a new command line option "-decorate".

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Closes: #1332