This option specifies the font to be used for the user interface where
a 'proportional' font is used.

A 'proportional' font may use a different amount of space for each
character.

This type of font is more esthetically pleasing than a 'monospace' font.

It is used by pfm for most parts of the user interface, except when a
multi-line text may contain columns that need to be aligned properly.
