TAB: improvements to fonts and presets

*) Add a serif font for fret marks (built-in FreeSerifMscore)
*) Use built-in FreeSans as fret mark sans font instead of special font
*) Split generic 6-str and 4-str presets into "simple" and "full" each
*) Remove "Bandurria" preset (now equal to 6-str full)
Merge pull request #256 from mgavioli/TAB_fonts_and_presets_updates

TAB: improvements to fonts and presets