Fix Double Click Detection

[TiFu]

This PR fixes the issues described in #66. This also fixes an additional issue:
If the user lifts the pen to start another stroke (e.g. when writing a T: first the vertical line, then the horizontal line) and less than the double click threshold milliseconds elapsed, the screen was cleared.

This is achieved by calculating the distance between the two click events: The screen is cleared if and only if the distance between two clicks is small (75 squared distance in this implementation) and the time between the clicks is less than the threshold (unchanged compared to master).

If you want me to refactor ̀isDoubleClick as discussed in #91, I can do that later this week (and, if wanted, also fix the setIsEmpty issue from #91).