Not all multi-finger touchpads are able to reliably produce gestures, so make
it optional. This patch just adds a boolean (currently always true) that gets
set on touchpad init time, i.e. it is not run-time configurable.

Three and four-finger gestures are filtered out in gesture_notify(), if the
cap isn't set the event is discarded.

For two-finger gestures we prevent a transition to PINCH, so we don't
inadvertently detect a pinch gesture and then not send events. This way, a 2fg
gesture is always scroll.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>