Choosing a parent class

[Andreas Kotowicz <kotoml mynetix de>]

I created a composite widget which consists of two frames, one of which
holds a calendar and a second one which has some buttons. I now ask
myself what is the right parent class to choose from. Is it GtkFrame?
Why can't I just choose GtkWidget? are there any rules which classes to
choose from for composite widgets? (I know, the tutorial says you should
use the class which is most similar but that's now very precise).

cheers,
Andreas