Functions creating a new GeglBuffer should trigger a warning if the
result if unused, because this is potentially a big memory leak.
Similarly objects created by functions creating new layers should be
handled (usually by adding the layer to the image with
gimp_image_insert_layer()), because they also come with a buffer and
possibly quite some important memory leak.
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.
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.
