convert as many "enum" as possible to "enum class", and make them ": char" in the due course #2

Follow up of PR 914.
Several enum types are "enum class" (i.e. typesafe C++ enums), several others are not.
Also making them ": char" should reduce the size of these structures.

Help in converting "enum ArticulationShowIn", "enum ElementFlag" and "enum Repeat" into an "enum class" needed...

Don't merge just yet, I'm still ploughing my way though the sources...