Add GesturesPropRegisterHandler

Add a function to GesturesPropProvider to allow registering:
 a "get" handler, called immediately before a GesturesProp is read
 a "set" handler, called immediately after a GesturesProp is updated

BUG=chromium-os:20768
TEST=compiles; unittests pass

Change-Id: I992c11a31fd0942cdfa75980c17bc55d5a0a9525