Use absolute coordinates for knob widgets, simplifying code
Whitespace cleanup
Added debug printout of resulting knob list
Add invalidation function for knob mappers
Added invalidation of knob mappings when mod select changed

* Intended firstly for Modulation Selection frames in UI, where changing the
  Select parameter must force a re-map of the knobs.
* Also useful if we chose to apply a similar hide/show functionality to the
  effects parameters.
Recalculate f_k_map when controller change recevied

* ... rather than when focus changed.
* This is due to GtkComboBoxes for some reason not emitting a
  focus-in-event signal, so focus is changed to a combo box, there is
  no way to map in the new knob map.
* Some caching going on though to avoid recalculating the whole knob
  map every time.
Implemented caching in blofeld_knobs()

* ... in order to avoid traversing the knoblist every time it is called.
* Note that we cannot cache it on a higher level, as we must consider
  the case of the knob map having been invalidated.
Configurable knobs-grab-focus in popup menu (by default enabled)