Automatically enable volume envelope whenever changing default envelope

### Enhancement Summary
Automatically turn AMT to 100 whenever a volume envelope is changed from the default.

#### Justification
New users to LMMS often do not realise the amount knob has to be turned. If AMT automatically moves to full when an envelope is changed from default, then we have a double advantage:
a) user need not look up how to enable envelope etc
b) user notices AMT knob going to hundred and can realise what it does.

The issue names no files, tests, or entry points. Start by tracing LMMS's volume-envelope and AMT controls, then verify that changing a default envelope sets AMT to 100 without altering unrelated behavior.

Written by the indexing model from the issue text.