core/window: change parent update signal to visibility change

Author: bbedward

src/window/floatingwindow.cpp

@@ -77,7 +77,7 @@ void ProxyFloatingWindow::setParentWindow(QObject* window) {
 
 		QObject::connect(
 		    this->mParentProxyWindow,
-		    &ProxyWindowBase::backingWindowChanged,
+		    &ProxyWindowBase::backerVisibilityChanged,
 		    this,
 		    &ProxyFloatingWindow::updateTransientParent
 		);