dynamic allocation of effect slots

[poisotf]

Supersedes #107
Following tests + USB capture of g923 on Windows, in an effort to match Windows driver behaviour :

This replaces the static allocation of hardware slots for particular effect types, with a "first come first served" scheme that can shuffle types.

There is also effect type fixup of FF_INERTIA and FF_FRICTION into "damper"

Different effects cannot be summed within one hardware slot, but several slots can be active with the same effect type.
On my unit, playing 3 slots of "damper" feels not great, but reasonable (correct result, with loss of precision and mild motor whine)

This should accept more diverse usage patterns from games before dropping effects.
#107 is already not dropping effects on any game I own, though.

I considered adding an extra gain knob for "inertia", but I'm not convinced this effect has a point. The "damper" gain is applied for now

[berarma]

Sorry for the delay. It looks good. I'll try to give it some testing soon.