Improve rejection of palm/edge touches

[aaronkollasch]

Jitouch contains code to reject palm and thumb touches, but it is not well adapted for current trackpad sizes, see #18 and #37.

This is a tracking issue for improving rejection of touches that graze the edge of the trackpad and register as an additional finger or fingers.

This could be done by adding a configurable dead zone around the trackpad where touches are ignored for gesture recognition,, a "virtual edge" of the trackpad, since most gestures will be in the middle of the trackpad. An exception would be left and right-side scroll as

Making this preference editable will make it easier for each user to adapt it to their own machine, as I can't test for values that make sense on every machine. It could still be hard to provide a sensible default for each machine, though perhaps I can use the reported trackpad dimensions to distinguish new and old style trackpads.