MidiPort: remove unneccessary line breaks
InstrumentMidiIOView: coding style improvements
MidiPort: introduced internal base velocity property

In order to keep compatibility with projects created with LMMS < 1.0.0
we maintain a property specifying the base velocity (i.e. the velocity
sent to MIDI-based instruments at volume=100%). For new projects this
always will be 64 while compat code enforces a value of 127 for old
projects.

We can also think about hiding the new groupbox in order to hide
complexity from the user.

Closes #430.