Also, use the model_column() internally for selection as well as for
painting to keep things consistent.
Note that we always fire the on_context_menu_request hook, even if you
didn't click on an item. In those cases, you get a GModelIndex().
This macro goes at the top of every CObject-derived class like so:
class SomeClass : public CObject {
C_OBJECT(SomeClass)
public:
...
At the moment, all it does is create an override for the class_name() getter
but in the future this will be used to automatically insert member functions
into these classes.
