gamepad: Make gamepad emit Event objects

Replace the 'button-event' and 'axis-event' signals from Gamepad by the
'button-press-event', 'button-release-event', 'axis-event' and
'hat-event' ones sending Event objects after mapping them is the Gamepad
object has a mapping, or directly if it doesn't.

https://bugzilla.gnome.org/show_bug.cgi?id=782611