Controller layout detection

[flibitijibibo]

The only weird-ish thing here is that we're using the Valve vendor ID to assign the Deck layout, but that technically applies to all virtual gamepads. This will eventually fix itself when we migrate to a system that uses Steam Input directly and we can get the hardware information again.

Fixes #945

[InfoTeddy]

So right now, my PowerA Xbox controller won't be detected as Xbox. Will that get fixed when we migrate to Steam Input?

[flibitijibibo]

That's the idea, yeah - at the same time, we can add to our own vendor/product database here as needed; normally I just fall back to Xbox and don't have a generic layout at all so I don't have a good database of Xbox vs. Generic.

[Daaaav]

This works well with both my PS4 controller as well as my Gembird (For True Gamers) Generic gamepad (it cost multiple dollars...)

If you'd like to fall back to Xbox instead of Generic that sounds fine by me - apparently it's more common to do so than I thought, and even Steam itself proudly displays even a big Xbox logo for cheap crappy generic controllers.

[flibitijibibo]

Alrighty, for now let's do that - generic will be worth keeping when we have more thorough hardware detection in the future.

(If anyone wants to beat me to this, feel free to push directly to this branch and merge, won't be able to push for a few hours)

[flibitijibibo]

Made the change to assume Xbox as the fallback, and added a vendor ID for 8BitDo devices. Will merge once CI passes!