gtkfont: refactor parse/unparse of XLFDs.

gtkfont: refactor parse/unparse of XLFDs.

There were already two places in the code (x11font_enum_fonts and
x11_guess_derived_font_name) where we retrieved an XLFD from the X
server, sawed it up ad-hoc into its '-'-separated parts and accessed
them by numeric index.

I'm about to add a third, so before I do, let's turn this into a
somewhat principled system where we get to do the decode/encode in
just one place and call all the individual fields by names that are
actually memorable.

No functional change intended by this commit.