[Development] Adoption of virtual_hook()

[jan-arve.saether at nokia.com]

The "Little Manual of API Design" [1] recommends adding a virtual_hook() that can be used later on if we need to extend the class without BIC. Lots of classes does not have this hook.

Isn't this something we should fix before Qt5 is out?

Jan-Arve

[1]: Little Manual of API Design, http://chaos.troll.no/~shausman/api-design/api-design.pdf