Pre-GIMP-3.0 code logics would re-allocate several GimpMenuFactory or
GimpUIManager for no good reason. While it was still working with old GtkAction
code, with our new GAction-based code, we were ending up overriding an action
with a new version of the same action, while keeping reference to old actions.
This made for discrepancies of the enabled or visible state of actions.
The new code keeps singleton of some objects and references to already
registered GimpUIManager or GimpActionGroups objects and make sure no actions
with the same name are created twice.