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...
Improve JPEG2000 error messages by using g_set_error() so we don't
throw many different errors in the users face, and make each error
unique and descriptive instead of using the same message regardless of
the type of error.
