Clarify MIDI CC menu ambiguity

Clarify MIDI CC menu ambiguity

CCs use 0-based counting, however CC 0 and 32 (bank select) are skipped
in Ardour's CC menu. This lead to some misleading enumeration.

Now, group CCs by 32, skip sub-menu if there are fewer than 32 named
items, and otherwise actual CC min/max numbers per named list.