Added HMD mount offset and convenience gesture typedefs

-If you tick auto-shift on hmd, this will automatically add the
eye->leapmount offset (8cm default value). Your vr hands should now be
completely 1:1
-Added Hand Type enum to Hand and relevant gestures. You can now easily
switch the hand type instead of checking isLeft/Right to reduce bp
clutter.
-Added  Leap Plugin Direction (Cardinal Direction) to gestures which
emit directions (Screen Tap, Key Tap, and Swipe). This is a convenience
property which will allow you to easily switch the property to get basic
swipe left/right/up/down/towards/away detection. If you need a more
precise direction, use the direction vector.