While looking for something unrelated I happened upon KDE documentation
for KAction or 'action patterns':

http://developer.kde.org/documentation/kde2arch/actionpattern.html
http://developer.kde.org/documentation/library/2.2-api/classref/kdeui/KAction.html

Something like these would seem a great enhancement for Gtk (or should
it be in GObject?)

Some things that would seem easier to do with this:
(In addition to those mentioned on the KDE pages.)
- OmniWeb (or NeXT? or MacOS?) style toolbar customization.
  This is where the app switches to a special mode that allows the user
  to drag icons representing predefined functions to the toolbar.
- Undo/Redo
  If the granularity is right, this should make keeping a history or
  making a history API simpler.
- Scripting
  Similar to a history I'd guess.

I'm going to try my hand at this. I'll start with something very similar
to KAction and then see what needs to be added or removed from there.
If anyone else is working on this, I'm particularly interested in being
able to use this from a lisp repl so that apropos calls can query
available actions.