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.
Fix symbols not read back from score file.

Symbols were not read back from score files: the 'sharp' glyph, used to initialize in Symbol() constructor, remained in effect after reading the symbol from score file.
Fix compatibility with 1.3 symbol names.

Symbols names stored in 1.3 scores (or previous 2.0 scores) are not recognized and replaced by a default fall-back symbol.

Added a vector and a Sym static function for re-mapping of old names into symbol IDs.

Also fixed:

- renamed "mensuralProlation1...11" to names expressing the meaning of the glyphs (in SymId, Sym::symNames, Sym::symUserNames and .json files);
- some details in fonts/bravura/glyphnames.json;
- added several glyphs to fonts/mscore/gplyhnames.json;
- corrected Sym::userName2id() to really return SymId::noSym on name not found,, rather than -1.
Merge pull request #553 from mgavioli/Fix_back-compatibility_wih_13_symbol_names

Fix back compatibility wih 1.3 symbol names
Merge pull request #551 from mgavioli/Fix_TAB_and_perc_clefs

Fix TAB and PERC clefs.