Remove plugins from FlxG/core.

...the user whether he wants to use a plugin and add it with `FlxG::addPlugin()`.

Gesture doesn’t work without adding the `GestureDetector` to `GdxStage._input`. `GdxInput` needs to extend `InputMultiplexer` to add other input processors like the gesture. I had to change `GdxInput` a little bit.

I’ve remove `Sensor` from the `system/input` folder and renamed to `SensorManager`. Previously it kept updating even if the app doesn’t need sensor. It’s now inside the plugin folder.

The others are `FlxTimer` and `DebugPathDisplay`, just add them if you need those classes.


**When you think this is ok and want to merge, also merge the `plugins` branch at flixel-gdx-examples.**