#38 Additional selection navigation features

The Audacity audio editor project recently added a
bucketfull of selection manipulation options, and
although their presentation is somewhat verbose, it's a
definite plus. I think it puts their selection control
nearly on par with The Gimp's. Here are some
suggestions for Cinelerra's selection control:
* When traversing labels with "Previous Label" and
"Next Label",
include the "In Point" and "Out Point" in the
traversal.
* Allow text to be associated with labels.
* Absolute positioning for track start/end, or a
label: ability to
type in a time or duration, or snap to a label.
* Right click on a track or effect and offer in the
pop-up menu:
Set In Point at track/effect begin
Set In Point at track/effect end
Set Out Point at track/effect begin
Set Out Point at track/effect end
Set active region to entire track/effect (OK, I
noticed that
double click on track does this. Redundancy is nice
sometimes. But an effect cannot be selected this
way.)
Set active region from project begin to
track/effect begin
Set active region from track/effect end to project end