IconView: set_cursor(): Add a version with no cell parameter
Update a TODO comment
Plug: Add documentation
Update a TODO comment
PrintOperation: run(): Add documentation.
Revert "Deprecate Gtk::ActionGroup, *Action, Activatable (sort of), UIManager."

Because we do not believe that the C API is ready to be deprecated.
See https://mail.gnome.org/archives/gtkmm-list/2013-October/msg00003.html

This reverts commit d1cfb5b5d805b2033334c8fb8845522e79333814.

We should apply this commit again some time in the future.
https://git.gnome.org/browse/gtkmm/commit/?id=d1cfb5b5d805b2033334c8fb8845522e79333814
Action, etc: Avoid C API deprecation warnings.