Change keyboard focus setting on visible/invisible events

Change keyboard focus setting on visible/invisible events

Keyboard focus was set and unset on activation events. This had the
side effect of keeping the keyboard attached to the window even if it
was invisible. Using the visibility events to perform this task looks
like a more natural way to deal with it.

Besides, activate/deactivate callbacks are not being called in
halibut or master, effectively stopping keyboard focus from working.

Bug-AGL: SPEC-2657

Signed-off-by: Jacobo Aragunde Pérez <jaragunde@igalia.com>
Change-Id: I65cc54c89db56bda1da4ea562151e3a30e10d068