xdg-shell: Clarify focus semantics for popup grabs

Make it clearer what the focus semantics are during a popup grab. In
short, when a grabbing popup is mapped, the top most popup will always
have keyboard focus, while pointer and touch focus works just as normal
except that only surfaces from the grabbing client will receive pointer
and touch focus.

This patch doesn't really change any semantics but rather clarifies
what was ambiguous before.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Mike Blumenkrantz <zmike@osg.samsung.com>