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:

- circle: The center symbol with Shift applied, with a fallback on Fn
- round trip: Same as the circle gesture but applied to a side symbol
- anticlockwise circle: Nothing currently. It is intended to be made
  configurable per-layout in the future.

The new Gesture class keeps track of what the pointer is doing while it moves
on a key. It replaces the 'selected_direction' integer.