show "secret" values, that is: values of nodes with children. These
can be used like all normal (leaf) properties, but their values were
until now only shown in writeProperties() dumps. Also show the
number of attached listeners. Example: foo = 'bar' (string, AU, L3).
dialog.cxx: add warning message for broken <format>s; cosmetics
there's a bug that I had copied: update() checks each list node: if it
has no children, then a listener is attached. Later, when freeing the
children list, it assumed again that each node without children would
have a listener attached. This caused a crash when a node had children
before, but lost them in the meantime. Now we tried to remove a listener
where there never was one.
