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.

as per UX

(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.

so we can style lockscreen, panel and favorites

so we can set it form settings later on

Make this a native package so we don't have to worry about patches,
tarballes, etc.