Fix cloning activity in sugar - SL #4608

Probably this code was not included in the initial port from gtk2 to
gtk3, because is only used by Sugar, but now we need it.

As a reference, the code is similar in sugar-toolkit:
https://git.sugarlabs.org/sugar-toolkit/mainline/blobs/master/src/sugar/activity/bundlebuilder.py#line67

Fixes #4608

Signed-off-by: Gonzalo Odiard <gonzalo@laptop.org>