FiguredBass navigation corrections

Remove empty FB element correctly in end edit.
Prevent Space and shift-Space from skipping FB not attached to note/rest.
Correct behaviour of Ctrl-number when changing to a new measure.
Call changeState() in the correct place - after startEdit().
A few comment corrections.
Check the track for a FiguredBass annotation.
Merge pull request #255 from softins/figured-bass-nav

Figured bass navigation corrections