This in turn needs xdg_shell from wayland-protocols

The last elements where outside the <interface/> block which
caused warnings.

Reviewed-by: Bob Ham <bob.ham@puri.sm>

will be tied to iio-sensor-proxy in the future

This adds a common base class PhoshMenu for all top panel menus so
phosh.c itself doesn't need to care about the details and we don't
duplicate the code.

(based on gnome-shell/maynard)

We switch to a private shell protocol from here on.  This might get
folded back into weston's desktop shell protocol at a later point when
we see what else is needed.