GtkUIManager has been deprecated without a good replacement for applications that want to keep a traditional UI (with a menubar, toolbar and statusbar). So I’ve written a new shared library called Amtk, currently developed inside the Tepl repository. It is a basic GtkUIManager replacement based on GAction. If you are interested, read the Amtk introduction (it explains the problems with what GTK+ currently provides and that Amtk solves) and the API reference.
Note that the library is not yet finished, factory functions are missing, release early release often etc. But I think that what remains to be done is not a lot of work (for my needs at least).