effects: Fix stacking order of windows relative to panels

Make windows thumbnails use their native stacking order as a 'z' value
for current desktop while lowering windows from other desktops.

By lowering others instead of raising current ones we get to use
stacking order compatible with desktop & dock/panel window types.

For now they are drawn separately, so this change doesn't do much on its
own, but it would become useful during refactoring when putting all
windows (regular and panel types) in a single parent with shared z stack.

src/effects/desktopgrid/qml/DesktopView.qml

@@ -75,9 +75,9 @@ FocusScope {
             wId: model.client.internalId
             x: model.client.x - targetScreen.geometry.x
             y: model.client.y - targetScreen.geometry.y
+            z: model.client.stackingOrder
             width: model.client.width
             height: model.client.height
-            z: model.client.stackingOrder
             opacity: model.client.dock ? desktopView.panelOpacity : 1
         }
     }

src/effects/overview/qml/ScreenView.qml

@@ -312,6 +312,7 @@ FocusScope {
             wId: model.client.internalId
             x: model.client.x - targetScreen.geometry.x
             y: model.client.y - targetScreen.geometry.y
+            z: model.client.stackingOrder
             width: model.client.width
             height: model.client.height
             opacity: container.effect.gestureInProgress

src/effects/private/qml/WindowHeapDelegate.qml

@@ -71,7 +71,7 @@ Item {
 
     visible: opacity > 0
     z: activeDragHandler.active ? 1000
-        : client.stackingOrder + (client.desktop === KWinComponents.Workspace.currentDesktop ? 100 : 0)
+        : client.stackingOrder * (client.desktop === KWinComponents.Workspace.currentDesktop ? 1 : 0.001)
 
     component TweenBehavior : Behavior {
         enabled: thumb.state !== "partial" && thumb.windowHeap.animationEnabled && !thumb.activeDragHandler.active

src/effects/windowview/qml/main.qml

@@ -208,6 +208,7 @@ Item {
             wId: model.client.internalId
             x: model.client.x - targetScreen.geometry.x
             y: model.client.y - targetScreen.geometry.y
+            z: model.client.stackingOrder
             visible: opacity > 0
             opacity: (model.client.hidden || container.organized) ? 0 : 1