add new gamecontroller APIs

- add mappings after init (or even before w/o using the hint)
- get string for axis
- get string for button
- get mapping string for controller or for GUID
- new event to notify when a controller is remapped. (e.g. mapping was changed via the AddMapping method)
add hint to allow disabling the use of Xinput.
Add SDL_TryLockMutex and implementations for all platforms