wayland: Fix the placement of popup windows

The fix in d61dde1 regressed the position of popup windows, since the
size was 0x0 when we wanted to do a sole move. Only fizzle out in the
path where we actually *do* resize.

https://bugzilla.gnome.org/show_bug.cgi?id=736812
wayland: Send accurate capabilities
window: Move set_custom_frame_extents to be X11-only

Wayland doesn't use custom frame extents anymore -- it uses a full
geometry description.
window: Don't queue move/resizes if the extents are the same

GTK+ sets the frame extents on every allocation, so don't bother doing
any extra work if things are the same.
window: Adjust the frame rect when _GTK_FRAME_EXTENTS is set on map