desktop: Make popup grab follow keyboard focus semantics

A popup grab is specified to have the top most popup surface gain keyboard focus. This means the keyboard focus should always follow the most recent xdg_popup.grab() surface. Make sure this happens by keeping track of the parent surface in the libweston-desktop popup grab, updating the keyboard focus when surfaces are added and removed from the popup chain, and restoring the keyboard focus to the toplevel when there are no popups anymore.

Signed-off-by: Jonas Ådahl jadahl@gmail.com