More convenient font list

[ewxrjk]

In rsbackup.5:

   Fonts
       FONT is a Pango font description.  The syntax is "[FAMILY-LIST] [STYLE-
       OPTIONS] [SIZE]" where:

       FAMILY-LIST
              A  comma-separate  list of font families.  These necessarily de‐
              pend on the fonts installed locally but Pango  recognizes  mono‐
              space, sans and and serif as generic family names.

              To get a list of Pango fonts:

                  python -c 'import pangocairo;print("\n".join([font.get_name() for font in pangocairo.cairo_font_map_get_default().list_families()]))'

The rune only works if you have the right Python version and modules installed.

Instead, rsbackup-graph could have an option to output a font list.