[WIP] Prepare 2.0.6

[zhuxingwei]

add the following two breaking changes to 2.0.6. We will wait for the release of the next version of Unity and merge this branch to the master then.

(1) RawTouch: we pass the raw touch events from engine to uiwidgets directly so that we can process user input better. (2) Vsync: we change vsync logics in both the Editor and the plugin side so that the frame duration and vsync timepoint is correctly calculated and triggered.