input: add support for precise scroll axes

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.
wayland: add support for precise scrolling

The default value for a standard mouse is 10.0. Because we don't want to
multiply the value in the input config file we scale it down to 1.0.

Hopefully this should work for more precise mousewheels or touchpad, but
I don't have access to such hardware.
input.conf: better documentation and sane defaults

Uses the same defaults as BTN3/4/5/6 which are hardcoded by most backends for
the mouse wheel.
changes.rst: add precise scrolling support