Epiphany, Galeon and probably others tabbed apps are able to reorder tabs
by dragging them around. I was wondering if it is possible (and if it makes
sense) pushing this feature down to the gtk+ notebbok widget.

The feature would be off by default, but could be turned on with something
like gtk_notebook_set_reorderable.
The idea behind this is that when you use a notebook for something like a
configuration dialog where each page is different the pages should not be
reorderable, while when a notebook is used as MDI (epiphany, gedit, ...)
being able to move the various tabs makes a lot of sense to me.

The obvious advantage is the reduction of code duplication and the fact
that apps that currently lack the feature would easily inherit it.