Repository/PluginSlot: Add ability to use next_class in the derived class

[shecken]

There is no way for plugins to route from their GUI to another one via the next_class system. The only chance so far is to read the next_class from the control structure again in the performCommand() and then use it. But that is not the purpose of this function. That is why I would like to add a performNextClass() that gives the plugins an option to react to the value in next_class.

The advantage is that you no longer have to write everything in the plugin's GUI and you get a clearer code.

[klees]

Addendum: I asked @shecken to fix CS, of course.