kiosk-shell allows xdg parent surface to obscure xdg children

To reproduce:

• Start weston-stacking in kiosk-shell.
• Create a new "transient" window by pressing the t key.
• Click on the parent window. Note how the parent window is raised to the top and obscures the child window.

The expected behavior is for the child window to remain on top, since it's an xdg child.

Note that to reproduce with latest weston, the fix for #679 (closed) needs to have been applied.