Layout issues in tab staves - chordline, glissando

Ubuntu Studio 14.04, GIT commit: 7eca27a

1) new score for tab instrument, 4/4
2) add some eighth notes
3) add a "scoop" before the second eighth in a group
4) add a gliss between third and fourth of group

Result: no additional space allocated for either. Scoop may thus collide with previous note, and gliss may be nearly invisible.

Similar issues already fixed for pitched staves, just need to copy the code.