Add the option to control the "speed factor" for thumb stick controls.

[duhrer]

Currently, we have a per-binding speed factor that controls how frequently a thumb stick setting repeats. We could add a setting to base the speed factor on the current deflection from neutral, so that pushing a stick all the way to the side repeats faster than a gentle deflection.

We should preserve the existing behaviour as an option as well, as some users may lack the precision to avoid fully depressing the stick, and might prefer a hard-coded and consistent value.

[duhrer]

This is slightly more complicated, as there are frequency settings (how often) and "factor" settings (how far in one go). The frequency setting can be incorporated into #108. The factor setting needs a little review, I'm willing to leave this one open for that work.