Fix popup positioning

The popup coordinates were being stored in an
unused variable and x,y offsets were left unmodified

Change-Id: Idbae7127b65963be6ddbe165d98ac7daa92cac9f
Reviewed-by: Pier Luigi Fiorini <pierluigi.fiorini@gmail.com>
Reviewed-by: Giulio Camuffo <giulio.camuffo@jollamobile.com>

Conflicts:
	src/compositor/wayland_wrapper/qwlshellsurface.cpp
	src/compositor/wayland_wrapper/qwlshellsurface_p.h