Regularise the mkdir and mkpath methods of QDir.

The mkdir() and mkpath() methods of QDir do not really make sense as they instance methods but take a parameter, even though QDir already represents a directory.

It is suggested that these methods are deprecated and replaced by an instance method which does not take a parameter and create the directory that the QDir object represents (perhaps create()) and static methods (createDir() and createPath()) which take parameters and allow the creation of arbitrary directories on the system without having to create a QDir instance.