[touch] make the swipe detection variance a multiple of the screen's DPI

The title says it all. The minimum distance a touch gesture has to move to be recognized as a swipe already is measured as a multiple of the screen's DPI but right now the allowed variance of the "transversal" movement of the gesture is hardcoded in pixels. This turns that variance value also into a multiple of the screen's DPI which should improve the swipe detection on screens with higher DPI.