Now InputDefault is responsible for giving out joypad device IDs to the platform, instead of each platform handling this itself.
This makes it possible for c++ modules to add their own "custom" gamepad devices, without the risk of messing up events in case the user also has regular gamepads attached (using the OS code).
For now, it's implemented for the main desktop platforms.
Possible targets for future work: android, uwp, javascript
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