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.