Remember groupFiles flag in pqFileDialog

pqFileDialog's constructor allows to pass an option to switch off file groupping. However, this flag was not persistent and was effective only on the first directory listing. Once the user changed the current directory, the flag was overwritten by the hard-coded default.

In this commit, the groupping flag is stored by the constructor within the implementation class and used afterwards in every call to pqFileDialog::pqImplementation::setCurrentPath.