Add some joystick functions to input. Enables manipulation of mapping…

Created by: Hinsbart

…s at runtime

The following functions were added to Input:

add_joy_mapping(String mapping, bool update_existing) and remove_joy_mapping(String guid) for manipulation of gamepad mappings.

bool is_joy_known(int device) defaults to true on all platforms that don't use the remapping code. Otherwise, checks whether the device is mapped.

get_joy_guid(int device) returns the sdl2 device guid if using remapping, defaults to "Default Gamepad"

I also documented these and the other joystick related functions. doctool updated quite a bit of things unrelated to this, I hope that's not a problem.

Also I hope the implementation is ok. I added is_joy_known and get_joy_guid to OS for the default variants. Platforms that use the remapping then override these to call back to InputDefault