allow disabling/enabling of menu entries via "enabled" property;

allow disabling/enabling of menu entries via "enabled" property;
Unfortunately, we don't have an easy way to access the puObjects
only by knowing the respective XML property node, because the
menu structure was built by plib from string lists. That's why
we walk the puMenuBar tree and store {property node}->{puObject*}
pairs in a map. With this infrastructure in place we can now
easily enable/disable entries, but we can also make other changes
to menu buttons as we see need. The structure of puMenuBar is
described in the pui documentation, so it's less of a hack than
it looks.  :-)