Documenting how to create and test a command that updates Java code

The idea is to document commands that add new code to existing classes (add commands). Example, the command to add a CDI producer or add a CDI injection point

Draft notes from #IRC

gastaldi UIContext.getInitialSelection() should return a UISelection of Resource objects
gastaldi Call get() on it and you should have a JavaResource if the command is executed in the context of a java class
gastaldi Make it the default value of the targetClass field

agoncal What does the Callable do in the initializeUI method do then ?
gastaldi That should be used when the value changes over the UI interaction
gastaldi That is not the case afaik
gastaldi Because once you execute the command, the initial selection will not change