Expose PopplerAnnot in the glib interface
Don't review yet, it's very primitive at the moment. I'm opening it in case someone wants to have a look early.
Right now I can ink annotations made of single paths in the glib demo. The result is not pretty, because the path is drawn using the raw points from the ink list. One should use the appearance of the annotation to have a better result (including supporting different stroke widths). The updating approach in the demo is quite naive: every time a new point is registered, the whole ink list is recreated in the underlying annotation. It's a bit laggy, but actually not so bad.
EDIT 13 May: an option was added to enable drawing more than one path on the same ink annotation. I changed the logic of the demo so that one can input more than one annotation (even of non ink type). When finished, the user should press the "Done" button to return to MODE_NORMAL.