New tablature features

New tablature features

this patch includes:
1) tablature is now "numbers only" as default
2) dead notes and palm mute is supported
3) "tied to" fret numbers become invisible
   (or parenthesized after line breaks or in a second volta)
4) the slope of glissando lines points in the right direction
5) grace notes (even parenthesized) are represented by smaller
   fret numbers.