Fix content type plugins to allow a default submit handler and default form like the new generic D7 one can.

To do this we need to detect if the basic form is needed: if override title is disallowed and there is no 'required context' we don't need a form. That means all the places we detect whether or not a form is needed will need to be aware of this as well, so we will have go genericize that.

This also needs to happen for D6, as this is a major DX WTF.