Pass item to rootObject() since it's now a singleton

The "parent" used to be resolved by the place where the utils.js was imported from.
However, it was changed into a singleton and now this didn't work.

BUG: 384776

src/declarativeimports/plasmacomponents/qml/Dialog.qml

@@ -230,7 +230,7 @@ Item {
         property bool loadCompleted: false
 
         Component.onCompleted: {
-            rootItem = Utils.rootObject()
+            rootItem = Utils.rootObject(this)
             loadCompleted = true
         }
 

src/declarativeimports/plasmacomponents/qml/Sheet.qml

@@ -277,7 +277,7 @@ Item {
         }
 
         Component.onCompleted: {
-            rootItem = Utils.rootObject()
+            rootItem = Utils.rootObject(this)
         }
     }
 }

src/declarativeimports/plasmacomponents/qml/private/AppManager.js

@@ -40,8 +40,8 @@
 
 .pragma library
 
-function rootObject() {
-    var next = parent
+function rootObject(item) {
+    var next = item.parent
     while (next && next.parent)
         next = next.parent
     return next