This function is not perfect and in particular doesn't seem usable with binding
because of GimpUnit being some weird mix between an enum and some kind of class.
So this will have to be fixed too. See #8900.
… 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.
