`GTabWidget` now allows the user/caller to get the currently active
tab widget, meaning that actions that are applied globally (such as
an 'Apply' or 'OK' button) can now react to specific tabs etc.
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.
