Fix gain inc/dec with multiple-selection spanning groups

Fix gain inc/dec with multiple-selection spanning groups

When tracks in a gain-sharing group are selected, stepping gain
up/down affected the tracks N times:
   for-each selected track inc/dec gain w/grouping.

When a mix of grouped and un-grouped tracks is selected, this lead to
inconsistent gain changes.

The new approach expands the groups first. Ignoring groups is not correct
either for single selection.