Support horizontal and vertical axes of input devices.
If the input device support precise scrolling with an input value then it
should first be scaled to a standard multiplier, where 1.0 is the default.
The multiplier will then applied to the following commands if possible:
* MP_CMD_SEEK
* MP_CMD_SPEED_MULT
* MP_CMD_ADD
All other commands will triggered on every axis event, without change the
values specified in the config file.