Use seat names as prefix, not as suffix, following
a top-down approach and avoiding issues
with applications that may be receiving
doubled signals (legacy + suffixed).
So instead of "mouse,in,seat1" signal will be
"seat,seat1,mouse,in".
If not provided, FOCUS_OBJECT action will keep acting
over default seat.
Also include a usage on edje-multiseat test
(actually no example was exercising this action).