… function gimp_font_get_pango_font_description().
Also updating file-pdf-save which is the only plug-in using these right now.
Note that I am not fully happy with the new function
gimp_font_get_pango_font_description() because I experienced some weird behavior
in file-pdf-save which is that some fonts were wrong if this is called after
pango_cairo_font_map_set_resolution().
But let's say this is a first step looking for improvements.
Now text layers are proper types, which means that the binding API will also be
nicer (e.g. `txt_layer.set_text('hello world')` in Python).
This commit also adds the param specs allowing to create plug-in procedures with
text layer parameters.
Finally it fixes the few calls in file-pdf-save (apparently the only plug-in
using specific text layer API right now) with explicit type conversion.
