Enhancement: command for fixed vertical spacing of lines in markup column

[Javier Ruiz-Alma]

RE: -user group inquiry
http://lists.gnu.org/archive/html/lilypond-user/2014-01/msg00672.html

The usage of this new command relates to creating LP score templates, which 
include a tagline having two stacked lines of text inside a markup block. The 
tagline markup block needs to have dimensional stability, even when users of 
the template choose to alter the default global staff size.

One can define fixed-fontsize markup for horizontal stability of text, but as 
of v2.18, the line-to-line vertical distance in markup blocks is scaled with 
staff size, and no apparent way to override this behavior from within the 
markup block itself.

The proposal is to add a command for this purpose.  Example:

\markup{
        \column{
                \abs-vertical-spacing #20  %value: points, mm, cm
                \abs-fontsize #15
                {"line1" "line2"}
        }
}