Adds input event generation for BTN_SIDE and BTN_EXTRA events to gtk and
input-linux methods.

Signed-off-by: Fabian Lesniak <fabian@lesniak-it.de>
Message-id: 20161206190007.7539-4-fabian@lesniak-it.de
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>