Pass correct MIDI channels to LV2 plugins

The channel number was getting stripped when sending MIDI data (both control and data) to a LV2 plugin.

- Control events: add channel number instead of `i` to the status
- Non-control events: re-add the channel to the MIDI event before sending (did this similar to CarlaEngineJack)
- The third commit shortens the code a bit by using ctrlEvent.convertToMidiData instead of duplicating code for every event type