Fix TAB and PERC clefs.

**TAB**: The two TAB clefs showed the same in the palette(s) and the 'flourished' one was displaying nothing in the score. Fixed by:

- adjusting TAB clef mapping in fonts/bravura/glyphnames.json,
- adding the proper mapping to fonts/mscore/glyphnames.json,
- correcting the symbol set for TAB in Clef::layout1()

**PERC**: The PERC2 clef has been dropped at some point in the past; it is no longer shown in the UI and silently rendered as PERC, if found in scores. Added some comments to:
- ClefType definition,
- Clef::layout1() function,
- to MuseScore::newClefPalette() function (with a reformulation of the _for_ instr.)

to make the situation more evident.

In practice, there almost no change in the code proper itself.