Fix selection of project files to add new files/classes to

Commit ac8e3714 did reorder the path list in
ProjectWizardPage, while ProjceFileWizardExtension still assumed
that the list displayed to the user is exactly the same as passed
to ProjectWizardPage.

The patch removes the index based communication between the
classes altogether and directly stores the ProjectNode in the
UserRole of the ComboBox entry.