The third version of xdg-shell introduces support for explicit popup
repositioning. If available, make use of this to implement popup
repositioning.
Note that this does *NOT* include atomic parent-child state
synchronization. For that,
https://gitlab.freedesktop.org/wayland/wayland-protocols/issues/13 will
be needed.
This currently uses my own fork of wayland-protocols which adds meson
support, so that we can use it as a subproject. Eventually when
wayland-protocols' meson support lands upstream, we should change it to
point there.
Silence some meson warnings while at it to make CI happy.
This also bumps the glib requirement, since g_warning_once() is used.
