Wizards: avoiding code duplication

I moved setPath(wizardDialogParameters.defaultPath()) into
the constructor of BaseProjectWizardDialog.

I created addExtensionPages() instead of having of having the
foreach everywhere. Moving the call into the constructor of
BaseProjectWizardDialog is not trivial since a lot of derived classes
rely on execution order and the order is often different.
Entangling this is not trivial and easily might break functionality.

Change-Id: I48dddaf72caea84da783dc9e2f42f2c7eff1c0ce
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>