Deduplicate drag handling from touch events

[paufau]

Fixes: #90

Solution:
Makes only actual screen InputEventScreenDrag to contribute to scrolling, omitting events from simulated mouse motion, so in this this condition:

if (event is InputEventScreenDrag and drag_with_touch) \
			or (event is InputEventMouseMotion and drag_with_mouse):
			or (event is InputEventMouseMotion and drag_with_mouse and not is_touch_dragging):

only (event is InputEventScreenDrag and drag_with_touch) is handling touch dragging. That fixes the duplicated handling

[paufau]

Hey @SpyrexDE, do you think it is possible to merge the fix?

Please let me know if it requires more thought or testing, I'd be happy to help