fontchooserwidget: Port to listmodels

fontchooserwidget: Port to listmodels

The port is kind of evil, in that it stores either a PangoFontFamily or a
PangoFontFace in the list, depending on if the fontchooser is configured
to select fonts or faces.
It also does not cache the font description anymore, so more calls to
pango_font_describe() may happen.

If both of these issues turn out problematic, the fontchooser would need
to resurrect GtkDelayedFontDescription again and put objects of that
type through the model.

These changes depend on Pango 1.46's introduction of listmodels and
various new getters, so the dependency has been upgraded.