gtkeventcontrollerscroll: Handle hold gestures

gtkeventcontrollerscroll: Handle hold gestures

Handle hold events:

 - GDK_TOUCHPAD_GESTURE_PHASE_BEGIN: scroll-begin is emitted.
 - GDK_TOUCHPAD_GESTURE_PHASE_END: A hold gesture ends only when all
   fingers are lifted from the touchpad without movement, so
   scroll-end is emitted right away.
 - GDK_TOUCHPAD_GESTURE_PHASE_CANCEL: A hold gesture is cancelled when
   some fingers are lifted/put down or movement is detected. In this
   case, scroll-end is emitted after a small timeout only if
   GDK_SCROLL wasn't detected.

Part-of: <!3454>