Feature transitions

See:
https://github.com/HaxeFlixel/flixel-addons/pull/97

This adds the ability to use transitions simply by calling FlxG.switchState(). It is using compiler conditionals so that there are no dependencies -- it automatically detects whether flixel_addons is in use.

(It also has a commit to fix the missing FillStyle typedef which was keeping flixel-addons from compiling)
Ah, I see. The problem with that is that the stateSwitched signal is not dispatched until after state destruction has already begun:

https://github.com/HaxeFlixel/flixel/blob/dev/flixel/FlxGame.hx#L609

The bitmap cache has been cleared, the cameras have been reset, etc. Furthermore, even then the original switchState() call will fall through after the signal logic resolves, destroying the state.

Or am I missing something?