TracksControl: Make the effectstack a child dialog.

This prevents the dialog from staying opened with the application
closed.

src/Gui/widgets/TrackControls.cpp

@@ -133,6 +133,6 @@ TrackControls::trackNameDoubleClicked()
 void
 TrackControls::fxButtonClicked()
 {
-    EffectStack *stack = new EffectStack( m_track->trackWorkflow() );
+    EffectStack *stack = new EffectStack( m_track->trackWorkflow(), this );
     stack->show();
 }