effects/overview: Load close button in WindowHeap on demand

For many windows it will be invisible, so there's little point for
creating it ahead of time. Another reason for loading the close button
on demand is that Plasma components are typically heavy.

src/effects/overview/qml/WindowHeap.qml

@@ -229,17 +229,19 @@ FocusScope {
                     }
                 }
 
-                PC3.Button {
+                Loader {
                     LayoutMirroring.enabled: Qt.application.layoutDirection == Qt.RightToLeft
-                    icon.name: "window-close"
+                    active: (hoverHandler.hovered || Kirigami.Settings.tabletMode || Kirigami.Settings.hasTransientTouchInput) && thumb.client.closeable && !dragHandler.active
                     anchors.right: thumbSource.right
                     anchors.rightMargin: PlasmaCore.Units.largeSpacing
                     anchors.top: thumbSource.top
                     anchors.topMargin: PlasmaCore.Units.largeSpacing
-                    implicitWidth: PlasmaCore.Units.iconSizes.medium
-                    implicitHeight: implicitWidth
-                    visible: (hovered || hoverHandler.hovered || Kirigami.Settings.tabletMode || Kirigami.Settings.hasTransientTouchInput) && thumb.client.closeable && !dragHandler.active
-                    onClicked: thumb.client.closeWindow();
+                    sourceComponent: PC3.Button {
+                        icon.name: "window-close"
+                        implicitWidth: PlasmaCore.Units.iconSizes.medium
+                        implicitHeight: implicitWidth
+                        onClicked: thumb.client.closeWindow();
+                    }
                 }
 
                 Component.onDestruction: {