Piano widget: fix velocity, Opulenz: increase volume

Take the velocity of piano key playing from the instrument's MIDI spinner. This way, if the user plays the instrument with the computer keyboard or with the mouse, the played sound is the same as 100% volume note.

Also increased the volume of Opulenz because it is very very quiet - abnormally so, to the point of having to add amplifiers to the chain just to get normal volume out of it, even at max velocity. May break backwards compat but Opulenz wasn't in the last official release, and it's better to correct this now rather than after 1.0.0.

Fixes #491