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.
