This solves an issue in GScrollableWidget where hiding one of the two
scrollbars needs to trigger a relayout since the other one should grow
into the "shared space" in the bottom right corner.
A GWidget can now override custom_layout() which will be called at any
time we would normally delegate work to the GLayout, e.g on resize
or child visibility changes, size policy changes, 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.
