use the correct image type
lots of commenting, early steps to handling missing segments
switch to new 16px icons
Merge branch 'master' into issue664_segments
handle empty track segments - sometimes this happens when a relative segment is about to be recalculated
handle empty track segments - sometimes this happens when a relative segment is about to be recalculated
introduce ability to watch for a segment's parent changing
handle before/after being deleted, tidy diagnostics