Some more introspection fixes

Take care of most of the warnings coming from the introspection scanner when dealing with Pango's sub-libraries.

The PangoFT2 namespace is particularly egregious, as it contains three different namespaces, only one of which uses pango_ft2 as the symbol prefix.

The remaining warnings after this merge request deal with missing types from fontconfig, freetype, and xlib introspection data.