shell: Change set_maximized to receive internal structures.

Change the parameters of set_maximized from wl_resource and wl_client to
shell_surface and weston_output. This will allow it to be used with
xdg-shell too.
weston: Add xdg-shell to the build.
xdg-shell: Implement part of the interface.

Basic requests are implemented, enough to get a surface displayed.
xdg-shell: Implementing maximized and fullscreen states.