Add comment about signal handlers' return value
Break out ui<->param value conversion to separate functions

* In preparation for new UI increment/decrement scheme.
Added param_update_value function for Blofeld and in param_handler struct
Properly handle updates of kaytrack values

* I.e. parameters whose displayed (UI) range is larger than the parameter
  value range; keytrack values are displayed as -200 .. +196 which is
  larger than the 0..127 of the actual parameter, so the increment in the
  UI is not 1 but larger.