Support multitouch

[shi-weili]

Currently, ga::TouchEvent doesn't track touch ID, which is needed to track individual touches.

Also, consider adding a toggle to turn on/off multitouch support. Not sure if that should be in GA::Kit, ofxGAkit, or an example app, though.

[tyhenry]

Good call -
I think multitouch id tracking for now would just mean adding an id field to ga::TouchEvent (it's actually in a comment as a reminder - just need to uncomment it!)

GAkit/src/ga/events.h

Line 65 in 57b58a1

int id;

[tyhenry]

Then we can extend the ga::TouchZone component and probably ga::Scene to toggle multitouch support at local component and global levels. We'll just need some logic to store / compare touch ids.