Add option to set c++ header file suffix

qdoc generates include statement in each class documentation which simply uses the class name. However qdoc could also be used for projects which don't use extensionless header files like Qt does.

It is possible to workaround with \inheaderfile but this needs to be added for every class. It would be nice if there was a global option to automatically add some extension, for example .h, .hpp, .hxx, .hh etc.