Create forward declaration headers for the Qt modules

This allows us more leniency in the future, as we can change between class / struct / typedef / namespaced names without breaking too much code, provided the forward declaration headers are used instead of manually forward declaring. Also encourages people not to include too many headers, which should improve compilation times.

Something like #include <QtGuiFwd> or #include <QtGuiForwardDeclarations> might be appropriate. The final naming needs to be decided.