gamepad triggers simulate mouse wheel up/down

[rumly111]

I tried playing with gamepad, but I missed the mouse scroll up/down feature.
I know you can press the dpad up/down, but you have to press/release/press/release
it many times to scroll up or down.
So I injected some code into gamepad_timer_fn() to check if triggers are pressed, and
send mouse wheel events if needed.

[rumly111]

I wrote the gamepad handling in CataclysmDDA game (see sdl_gamepad.cpp), and I wanted to use similar approach here, but that would require rewriting a bunch of stuff,
so I changed my mind. Let me know if you think my added code is too ugly, and I'll try to think of something.

[rumly111]

thanks