Jokosher trunk already uses GtkBuilder in trunk. This bug is set to milestone 0.12 and that's the version it will be release in. We just haven't got around to making a release in a while. Sorry that the status of this bug as not been updated to fix committed.
Revision 1113 in trunk merged the GtkBuilder for Jokosher.
Revision 1116 in trunk merged the GtkBuilder changes for the extensions.
Also for your second question, the python version number just reflects the version of setuptools that they were compiled for. The code works with all versions of Python that Jokosher supports. The sources are in trunk under extensions/eggs/ and can be compiled for whatever version of setuptools is available on the user's system.