Fix some interpolated values of a trackpoint on insertion between points.

Fairly obvious cut and paste error.
Incorrect code introduced (by me) in commit SHA1: db79f75f7deef5e8a934324c69dfaa274764514a
Enable control of create_label_vbox spacing and padding.
Add view of course and speed properties of trackpoints.

Since these values may be held in the trackpoint data, we might as well be able to show them!

Also reduce spacing between the elements to keep the dialog compact.