As we now use partial classes for custom code, we can put additional
interface implementation declaration in the custom code, instead of
adding it through a fix-up.
Side-note: I thought about moving to IEnumerable<T>, but ListStore is a
list of objects anyway, and Container is an array of Widgets, so there
wouldn't be much benefit from type safety.
There are no real code changes in this commit, just a lot of file
renaming and boilerplate additions.
A few .custom files are just removed, because the corresponding class in
GTK is gone, so they were not really used anymore.
Some files need to be re-indented, but that will be done in a separate
commit, so that git can track the renamed files correctly and not be
confused by all the changes.
