Prevent diagonal drag/flick

Usually, you want to scroll through the programs for a given time (horizontally) or through time for a given program (vertically). Therefore, movement in horizontal and vertical direction at the same time should be avoided.

Thresholds are needed to avoid that small movement in one direction locks the direction even though the main direction of the movement would be the other one. However, this leads to some more or less random magic numbers which may or may not work, which could cause incomprehensible UI behavior.