theme-node: Fix font family formatting

theme-node: Fix font family formatting

Unlike pango_font_description_from_string(),
pango_font_description_set_family() requires a already properly
formatted font family string. The proper format is a comma seperated
list of font families, but we generated a "comma space" separated list.
Passing a incorrectly formatted font family string to pango seems to
cause wierd issues, where the wrong font is sometimes selected.

For example, this fixes a font selection issue on zh_TW.UTF-8 locale for
chinese characters, where previously the "Droid Sans" font was selected
instead of "Source Han Sans TW" even though fontconfig had placed
"Source Han Sans TW" before "Droid Sans".

https://bugzilla.gnome.org/show_bug.cgi?id=786868