This notifies the midi notes to the editor, and shows them in VK accordingly.
The notifications are pairs (key, velocity) where velocity=0 means off.

There is a problem about doing this feature in LV2, which I have trouble to solve.
When writing these notification messages, LV2 UI is not receiving all of them, leading to stuck keys display.
That's although the atom forge reports no error writing the buffer.
Another curiosity which I noticed, output control ports are notifying to UI non-stop, although the value is not changed.