server: Fix crash with popup menus

With Wayland 1.2 there was only one resource so we could call
wl_resource_get_client without problems, however with Wayland 1.3 we
have to go through a list of resources but we called
wl_resource_get_client on an invalid resource so we retrieve the display
pointer from the seat before the resource iteration.

Closes #115
client: Simplify background

Don't swap between two background renderers, just put all the background
elements together and change the visible property according to the
settings.
Add copyright information to the hawaii.xml protocol file