Extending the plugin API and callbacks API without breaking the ABI

[jrohel]

Plugin hooks and callback functions are done using virtual methods. Callback users and plugin developers implement the necessary virtual methods. This approach is simple and, moreover, supported in SWIG. SWIG supports cross-language polymorphism - SWIG directors.

During development, as in this PR #1592, there may be a need to extend the API - add a new plugin hooks and a new callbacks. The problem is that adding a new virtual method to an existing class will break its ABI.

A technique needs to be found for adding new plugin hooks and callbacks without breaking the original ABI. The solution needs to work with SWIG.