WIP: Make group edit page a subclass of scrolled window
Rather than having a single weird GroupEdit page object in our
UnlockedDatabase all the time, that we "fill" the properties of
the group in, make GroupEditPage a real Subclass of ScrolledWindow.
This way all the logic for the Group Edit Page can be cleanly encapsulated
in this class and the horrible show_page_for_new_groups() function does not
have to contain all the logic for creating all kinds of pages.
A similar thing could be done to the EditPage later