overrides for message-dialog, window, tree-view and tree-store with relatives

With this I provide all overrides necessary to migrate all existing opine widgets to gir-ffi-gtk3.
You may want to give me commit access to the repository as I expect to maintain much of the codebase while giving life to opine.

Enjoy,
Patrick

Hah, well, if the LGPL is holding you back, I'm wondering why you're using Gtk+ :laughing:. Anyway, I'll close this pull request then.