WinRT: more keyboard code cleanups
- made keycode listings a bit easier to read
- listed VK_ keycode names, where appropriate
- removed a few pieces of dead + commented-out code
- applied a common 'WINRT_' prefix to internal function names
Fixed wording in SDL_GameControllerAddMappingsFromRW() error message.
Removed two wrong documentation comments from gamecontroller source.

No replacement because correct documentation is already in SDL_gamecontroller.h.
Added handling of NULL as input for SDL_GameControllerMapping().

For consistency with the similar functions getting SDL_GameController as input.
Also NULL is no SDL_GameController and therefore can not have a mapping anyway.