pointer-gestures: add hold gestures

Hold gestures merely denote "there are fingers on the touchpad but they are
not moving". As touchpad touches are generally fully abstracted, a client
cannot currently know when a user is interacting with the touchpad without
moving - no motion events will be sent in this case.

The two use-cases here are:
- hold-to-interact: where a hold gesture is active for some time
  a menu could pop up, or some object is selected, etc.
- hold-to-cancel: where e.g. kinetic scrolling is currently active, the start
  of a hold gesture can be used to stop the scroll

Since hold gestures by definition do not have movement, there is no need for
an "update" stage in the gesture.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>