Qt: add a tooltip for people who don't get that save will close the dialog.

modules/gui/qt4/dialogs/preferences.cpp

@@ -81,6 +81,7 @@ PrefsDialog::PrefsDialog( QWidget *parent, intf_thread_t *_p_intf )
     /* Buttons */
     QDialogButtonBox *buttonsBox = new QDialogButtonBox();
     QPushButton *save = new QPushButton( qtr( "&Save" ) );
+    save->setToolTip( qtr( "Save and close the dialog" ) );
     QPushButton *cancel = new QPushButton( qtr( "&Cancel" ) );
     QPushButton *reset = new QPushButton( qtr( "&Reset Preferences" ) );