Rel inputs (rel2rel, rel2btns, rels2axes), and joystick calibration

[yyon]

Hello!
I was trying out your project and found it was missing some functionality I wanted, so I tried implementing it.

I use a controller similar to the Azeron controller, which is designed to be used with a mouse, so I wanted to be able to remap the mouse, but MoltenGamepad was missing rel inputs, which prevented me from using the mouse movement. I added in the ability to use rel inputs, and added the following translators which use it:

• rel2rel: maps a rel event to a rel event. I needed it so I could block the mouse inputs while still using the mouse movement (needed for some games).
• rel2btns: maps a rel event to two buttons: one button is pressed if the input is moved positive, the other if it is moved negative. Used so I can remap the mouse wheel up/down to two buttons.
• rel2axes: maps two rels to a joystick. Used so I can control the camera with a mouse on games which don't support mixed mouse+keyboard and gamepad inputs, when I want to send gamepad inputs instead of mouse+keyboard.

There are two other small changes that I added. One is the ability to calibrate an uncalibrated joystick with some additional optional arguments for axis2axis. The other is adding the scroll wheel events to the virtual mouse output.

I don't imagine that using an Azeron-like gamepad with a mouse is that popular of a use case, but hopefully it's popular enough to warrant the inclusion of this pull request. I don't often send pull requests, so let me know if I did anything wrong, or if you need anything more (more documentation, code changes, etc.).

[Arctic-Circle-System]

I'm pretty sure at this point, you'd be better off forking the project. The dev hasn't been active on GitHub for years. For all I know, they might be dead.