Like most other cvars 'sensitivity' allowes for float values. But until
now mouse events were handled as integers which led to some confusing
problems. This was especially noticeable at values lower than 1, small
mouse movements were cut to 0 and discarded. Since the clients movement
code is written in floats and we're already using floats for joystick
movement switch the mouse event handling over to them, too.
This should have any impact on configs were 'sensitivity' is ste to
integral values. If it was set to decimal values the behaviour is now
correct.
This fixes#419.
There's an "enable alt joy keys" command now. If a key is bound to that
command, all joystick buttons (incl. hat and triggers) are turned from
K_JOYx into K_JOYx_ALT, which allows two keybindings on the same key,
one with the altselector pressed and one without.
If there's no keybinding for K_JOYx_ALT, it will use the binding for
just K_JOYx as a fallback (if it exists).
This is especially handy to create direct bindings for all the weapons
on the (limited amount of) Joystick buttons.
The input system backend was once used in the client and the renderers,
but for some years now it has been an integral part of the client only.
Move it there.
