window: Improve how we tell the window to do layer-shell

Instead of telling it in the construction after forcing the window
creation, install an event handler that sets it when we get the
PlatformSurface event.

It has the advantage that it will also trigger in subsequent platform
surface events (e.g. after hide and show).

tests/become-layershell.qml

@@ -0,0 +1,42 @@
+/*
+ *   SPDX-FileCopyrightText: 2025 Aleix Pol i Gonzalez <aleixpol@kde.org>
+ *
+ *   SPDX-License-Identifier: LGPL-3.0-or-later
+ */
+
+import QtQuick
+import QtQuick.Controls
+import org.kde.layershell 1.0 as LayerShell
+
+Item
+{
+    Button {
+        text: "Convert"
+        anchors.centerIn: parent
+
+        onClicked: {
+            win.close()
+            win.LayerShell.Window.anchors = LayerShell.Window.AnchorLeft;
+            win.LayerShell.Window.layer = LayerShell.Window.LayerTop;
+            win.LayerShell.Window.exclusionZone = -1;
+            win.show()
+        }
+    }
+
+    Window {
+        id: win
+
+        width: 100
+        height: 100
+        Rectangle {
+            anchors.fill: parent
+            color: "red"
+
+            Text {
+                anchors.centerIn: parent
+                text: "top"
+            }
+        }
+        visible: true
+    }
+}