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 and adding the proper mapping to fonts/mscore/glyphnames.json and corrected the symbol set for TAB in Clef::layout1()

PERC: The PERC2 clef has been dropped at some point in the past: added some comments to ClefType definition, to Clef::layout1() function and to MuseScore::newClefPalette() function to make the situation more evident.

In practice, there almost no change in the code proper itself.
Merge pull request #551 from mgavioli/Fix_TAB_and_perc_clefs

Fix TAB and PERC clefs.