This change reshuffles the way that parameter updates and notifications
work for hosted VST3 plugins.

Previously:
- Parameter::setValue would update the processor and editor, but not the
  host
- Parameter::setValueFromEditor would update the processor and host, but
  not the editor
- MIDI CC events would be converted to events and added to the
  processor's input event list, but were not forwarded to the
  IEditController.

Now:
- Parameter::setValue updates the host's cachedParamValues, which is the
  host's source of truth for parameter values. On each process callback,
  changes will be added to the input parameter list. Then, for each item
  in the parameter list, an update will be dispatched back to the
  editor.
- Parameter::setValueFromEditor is removed. All parameter changes will
  be sent back to the editor, even if they originated from the editor.
  setValueNotifyingHost can be used to notify listeners that one of the
  host's JUCE parameters has changed, e.g. in the host context's
  performEdit.
- MIDI CC events trigger calls to setValueNotifyingHost on any mapped
  parameters. The flow is very similar to a parameter change from the
  editor: the cachedParamValues are updated immediately, and host
  parameter listeners are notified. Then, for each changed
  cachedParamValue an entry is added to the inputParameterChanges
  (updating the processor), then for each item in the
  inputParameterChanges an update is sent to the editor.