[Qt] delete Qt preferences too when resetting prefs.

Close #2126
(cherry picked from commit ec8639f0)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/gui/qt4/dialogs/preferences.cpp

@@ -360,9 +360,10 @@ void PrefsDialog::reset()
     {
         config_ResetAll( p_intf );
         config_SaveConfigFile( p_intf, NULL );
+        getSettings()->clear();
 
-        instance = NULL;
         close();
+        instance = NULL;
         PrefsDialog::getInstance( p_intf )->show();
     }
 }