core/window: add parentWindow property to FloatingWindow

src/window/test/manual/parentwindow.qml

@@ -0,0 +1,42 @@
+import QtQuick
+import QtQuick.Layouts
+import QtQuick.Controls.Fusion
+import Quickshell
+
+Scope {
+    FloatingWindow {
+        id: control
+        color: contentItem.palette.window
+        ColumnLayout {
+            CheckBox {
+                id: parentCb
+                text: "Show parent"
+            }
+
+            CheckBox {
+                id: dialogCb
+                text: "Show dialog"
+            }
+        }
+    }
+
+    FloatingWindow {
+        id: parentw
+        Text {
+            text: "parent"
+        }
+        visible: parentCb.checked
+        color: contentItem.palette.window
+
+        FloatingWindow {
+            id: dialog
+            parentWindow: parentw
+            visible: dialogCb.checked
+            color: contentItem.palette.window
+
+            Text {
+                text: "dialog"
+            }
+        }
+    }
+}