[kaccess] Revamp default values for mouse key acceleration parameters

Currently some of the default values are computed based on the current value of other parameters

This is somewhat awkward to properly reflect in the UI

It also results in somewhat nonsensical values and a very steep acceleration curve

Instead use default values for all parameters, based on example values from the XKB reference

As a result there is now an actually noticable acceleration curve instead of reaching the max speed immediately