wheel: ignore initial small scroll deltas

wheel: ignore initial small scroll deltas

Mice with high-resolution support can generate deltas when the finger is
put on the wheel or when the user tries to click the wheel.

To avoid sending involuntary scroll events, add an extra state the the
wheel state machine to accumulate scroll deltas.
While the accumulated scroll is lower than a certain threshold, ignore
them until the threshold is reached.

Since no finish event is sent by the mouse, reset the state machine
after a period of scroll inactivity.

Signed-off-by: José Expósito <jose.exposito89@gmail.com>