Standardize buttons across multiple platforms/controller types

[luanpotter]

We need some generic way of identifying buttons in a platform and controller independent way
Also standardize dead zones and intensities of the triggers

Related: #4

[markvideon]

This might be a useful resource.

[markvideon]

I imagine this would look like defining some common set of controller elements inside gamepads_platform_interface and then each plugin implementation mapping inputs to those before invoking onGamepadEvent.

I think Unity is a good reference - using BUTTON_NORTH, BUTTON_SOUTH, etc. rather than BUTTON_X to describe a particular button on a controller, as the names of the buttons should not be considered uniform among all possible controllers. I'm less familiar with the input APIs of other game engines.