Add controller support

[stackotter]

This feature request is pretty self explanatory. Basically just use Apple's game controller framework to add controller support to Delta Client on Apple platforms. It would especially nice for Delta Client on iOS. (Thanks @ninjadev64 for the idea ages ago and thanks @JWhitmore1 for reminding me)

[stackotter]

I've basically implemented the input side of things, now all that remains is to handle controller switching and show the user visual confirmation once a controller is detected/connected and let them know when it disconnects. We don't need to implement input remapping because System Settings provides a nice way for users to do that per app.