Xbox Controller Updates

[StephenHodgson]

• Addresses some feedback from @keveleigh.
• Adds a None enum to the XboxControllerMappingTypes that will break all existing settings.
  • Sorry, None should have been there from the beginning!
• Added a XboxControllerHanderBase for easily creating your own implementations.
  • Handy helper functions for getting buttons based on enum to change/update inputs on the fly.
• Added optional parameters to XboxControllerSource when overriding events on the StandaloneInputModule

[SimonDarksideJ]

Was looking at how to bring this forward to the Develop2017.2 branch, however these classes have been removed there :S

[keveleigh]

@DDReaper These classes never existed there; these are new, added to master in #936. The Dev branch has an implementation of Gamepad input, mostly in GamepadInput.cs, which added "gesture" support via the gamepad, but not discrete button mappings.

[SimonDarksideJ]

Ok, I'll check that out, as I'm getting no input from the Wireless Original Xbox One controller currently @keveleigh (Just tested your PR)
Controller works fine in MRPortal.

[StephenHodgson]

@DDReaper, did you implement the InputManager.asset?

[SimonDarksideJ]

Ok, figured out the problem. As noted in #964 , the base dev branch is broken for Immersive headsets.
Headmovement and gaze functions, but the MR camera is floating and Xbox controllers don't work. Fixed by pulling in @keveleigh s fixes

[StephenHodgson]

@DDReaper this PR has nothing to do with the dev branch.