indicators: Update the indicator's visibility in the base class

So derived classes don't have to.
Doesn't affect ClassicIndicators, since it uses a separate
window for them. But useful for new indicator types.

src/private/DropIndicatorOverlayInterface.cpp

@@ -38,6 +38,7 @@ void DropIndicatorOverlayInterface::setWindowBeingDragged(const FloatingWindow *
         setHoveredFrame(nullptr);
     }
 
+    setVisible(m_windowBeingDragged != nullptr);
     updateVisibility();
 }