Support for motion in evdev joystick backend

[v1993]

Right now, SDL only supports motion input when using hidraw. This is troublesome for applications that
rely heavily on motion and want to work on Linux, since a) hidraw isn't enabled by default on Linux b) there are good reasons behind that, involving kernel drivers and permission issues.

On a plus side, evdev totally supports motion input and joystick drivers make use of this interface. On a downside, motion input is exposed as separate device from main joystick and there isn't a standard axis mapping (it varies per joystick, so some kind of database is needed).

Right now, SDL just exposes those extra motion devices as joysticks, causing some confusion. A possibility would be to check if device have INPUT_PROP_ACCELEROMETER property and use them as motion source for device that have no such flag set (main joystick device) and have same non-empty uniq value.

I'm by no means knowledgeable about SDL internals, but feel free to ask me about motion via evdev, I'll try to help.

[slouken]

This will be implemented in #6478, thanks!