Issue 3860: Add command \justify-line

The command \fill-line spaces words taking word length into account
such that the following distances are equal: outside edge of outside
markup to center of adjoining markup; center to center of
neighboring interior markups.  As a consequence, the space between
words is unequal when the lengths of the markups vary and/or there
are more than three markups in the line.

The following patch creates the command \justify-line, which spaces
markups to fill a line such that the space in between each pair is
constant.  Collisions are prevented by ensuring that this distance
does not fall below the current setting of word-space.
Release: bump VERSION_DEVEL.
PO: update template.
Release: update news.
Release: bump VERSION.
Fix issue 3868: line-breaking makes short lines

Two simple fixes:
1) Always have some stretchability between notes
2) If there _is_ a completely unstretchable line, shorter than the line-width,
   score it as a bad fit, not as a perfect fit (report large forces for stiff lines).

Either one would fix the problem; both seem to be good to have in the code.