Separate gamepad from mouse sensitivity

* As before, when using a gamepad, "mouse sensitivity" becomes "gamepad
sensitivity" in the options menu, but now mouse and gamepad sensitivity
are independent from each other. A new "gamepadsensitivity" setting is
in doomretro.cfg, allowing even values between 0 and 128 inclusive, with
a default of 128.
* The overall sensitivity of gamepads has been reduced.
* The logic in making sure "mousesensitivity" in doomretro.cfg is an
even number has been fixed.