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.
Jehan