remove unnamed enum { SHOW }

as it was use in only 3 places in 2 of which a simple 'if/else' seems
the better choice and in the 3rd a simple '0'.
convert unnamed enum into enum class GripLine
convert unnamed enum into enum class GripSlurSegment
convert enum NoteSpellingType into enum class
convert unnamed enum into enum class Prefer
convert unnamed enum into enum Tpc

but haven't had the guts to make it an enum class yet
convert unnamed enum into enum class A

and cleanup enum Repeat, a leftover from 6c6c6ad
convert unnamed enum into enum Key

but converting into enum class requires some more work. And mostly for
that reason can't yet get rid of the "KEY_", as it clashes with enum Tpc
(which too hasn't yet been converted into en enum class)
minor cleanup after e567406
convert enum LoopType into enum class
convert enum VeloType into enum class
convert enum TempoType into enum class
convert enum TrillType into enum class
convert enum TimeSigType into enum class

and fix the special handling of old style common- and cut time
convert enum TremoloType into enum class

and fix a minor white-space damage caused by 0f28d75 and 2ae0940
convert unnamed enums into enum class NumberTyp

resp. enum class BracketType
convert enum HookType into class enum
convert enum SpacerType into enum class
convert enum SlurAnchor into enum class
make enum SegmentType an unsigned short

more work would be needed to make it an enum class, unfortunatly, as it
is used as flags