fontchooserwidget: Refactor code so we actually optimize

Previously, we would pango_font_describe() every time the code ran and
we wouldn't ever hit the optimized quick exit.

The code now is a lot more complex because the
compute-actual-value-when-required-the-first-time approach is not
supported out of the box in GtkTreeModel (or GValue).

gtk/ui/gtkfontchooserwidget.ui

@@ -8,7 +8,7 @@
       <!-- column-name face -->
       <column type="PangoFontFace"/>
       <!-- column-name description -->
-      <column type="PangoFontDescription"/>
+      <column type="GtkDelayedFontDescription"/>
       <!-- column-name preview-title -->
       <column type="gchararray"/>
     </columns>