Handle key press and release correctly for buttons

The clicked signal should not be emitted multiple times when the
button is just pressed.

Change-Id: I3f1f3ab7f135b366150af985b41409afa0130568
Reviewed-by: Leonardo Sobral Cunha <leo.cunha@nokia.com>
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@nokia.com>