Create some base classes for plugin development

There are some core actions a plugin that creates Java classes must do, such as get the current package. Actions like this should be on an abstract base class such as JavaSourceCreationPluginBase (maybe something shorter?) which would also implement Plugin (so we don't break existing plugins) that will have these methods baked in so each plugin doesn't end up copying the code from somewhere else. If Java had mixins this would be a perfect fit, alas, it does not and we're stuck with something else.

The class would probably have to have some abstract getter methods that the subclass would have to implement to get common things like the project.