This implements clockwise/anticlockwise circle and round trip gestures
inspired by Messagease.
The circle gestures start after a small threshold to avoid making the
regular swipe too hard to aim.
The gestures do:
- clockwise circle: Shift variant of the center symbol
- anticlockwise circle: Fn variant of the center symbol
- round trip: Fn variant of the targetted side symbol
The Gesture class now keep track of what the pointer is doing while it
moves on a key. It replaces the 'selected_direction' integer.
