Fix the popups handling.

Use wl_shell_surface_set_popup() instead of wl_shell_surface_set_transient(),
since the latter is not supposed to make the surface follow the transient
parent movements.

Needs Weston master or 1.0.6, otherwise freezes will happen.
To have the best experience you should however use recent weston master.
The popups in QtCompositor are badly placed.

Change-Id: I3c86e5d74503514749840681b4e81c58cd73ca70
Reviewed-by: Andy Nichols <andy.nichols@digia.com>