[rotation] simplify rotation usage by introducing rotationParent property

src/qml/GlacierRotation.qml

@@ -41,6 +41,12 @@ Item {
         property bool nativeIsPortrait: ((nativeRotation === 0) || (nativeRotation === 180))
     }
 
+    property Item rotationParent
+
+    function rotateRotationParent(o) {
+        rotateObject(rotationParent, o)
+    }
+
     function rotateObject(obj, o) {
 
         var r = Screen.angleBetween(o, Screen.primaryOrientation)

src/qml/MainScreen.qml

@@ -72,16 +72,17 @@ Page {
 
     GlacierRotation {
         id: glacierRotation
+        rotationParent: desktop.parent
     }
 
     orientation: Lipstick.compositor.screenOrientation
 
     onOrientationChanged: {
-        glacierRotation.rotateObject(desktop.parent, orientation)
+        glacierRotation.rotateRotationParent(orientation)
     }
 
     onParentChanged: {
-        glacierRotation.rotateObject(desktop.parent, nativeOrientation)
+        glacierRotation.rotateRotationParent(nativeOrientation)
     }
 
     Component.onCompleted: {