Normalize potmeter widget values to [0.0, 1.0] instead of [0.0, 127.0].

This is another step towards getting rid of MIDI assumptions that are baked into
the widget system.

* Control parameter space for potmeters is now [0.0, 1.0] instead of [0.0,
  127.0].
* Non-linearity of parameter space (64 being the center value) now only applies
  to MIDI, not widgets.
* Fix all potmeter-style widgets to use [0.0, 1.0] for parameter values.
Fix seek min value check.