Fix #22996 - Wrong clef when selecting standard staff type in instrument wizard.

In the instrument wizard, when a staff is changed to standard type (from a previous different type), the standard clef of the istrument is lost.

Changes:
- Added a member variable to `StaffListItem` storing the default standard clef for the instrument.
- Modified `instruments.xml' to remove &lt;clef&gt; tags from tablature instrument definitions: all these instruments are inited from a standard-staff main entry, recording the default standard clef; this clef is used to init the `StaffListItem::_defaultClef` member variable.

NOTE: Percussion instruments are not covered (an number of percussion instruments seem to have old-style definitions in `instruments.xml`); conversion from percussion to standard staff may yield unexpected results.
In instruments.xml, fix percussion instruments to use &lt;stafftype&gt; tag.
Disable stadard staff support for percussion staff types
Fix init of group combo in Staff type editor
fix #20579
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