instead of gtk_container_add()'s because otherwise the widget will not expand in GTK+ 3.x
I'm sure some plug-ins need to add their items *not* at the toplevel, but since making plug-ins really tree-aware is a lot more work than just fixing insert positions, I went for passing -1 as parent in almost all cases. And because of laziness...
Renaming the PDF import plugin to file-pdf load is needed in order to introduce the new PDF export plugin