Deprecate Variable Listeners

[triceo]

Convert all internal uses of Variable Listeners to the new mechanism, then deprecate Variable Listeners for removal. This should include deprecating any annotation which is implemented via a user providing a Variable Listener.

In general, the supported types of embedded shadow variables should be reviewed to find out which are still needed, and which may be deprecated. We need to decide what to do with:

• Cascade shadow var. (Can it do something that DSV can not? If not, deprecate.)
• Piggyback shadow var. (Can it be connected to DSV? Does it have any other uses?)

In order to deprecate var listeners, Python people will need an alternative. One part of this task therefore needs to be Python support for DSVs.