QmlDesigner.PropertyEditor: Use selectionToBeChanged in LineEdit

Commit any data to the backend if the selection will change.
The signal editingFinished is usually triggered to late.

Change-Id: Ib7adfd32a50ae94b017a7587e81721176440a988
Reviewed-by: Marco Bubke <marco.bubke@digia.com>
Reviewed-by: Tim Jenssen <tim.jenssen@digia.com>

share/qtcreator/qmldesigner/propertyEditorQmlSources/HelperWidgets/LineEdit.qml

@@ -74,6 +74,17 @@ Controls.TextField {
     onTextChanged: {
         __dirty = true
     }
+
+    Connections {
+        target: modelNodeBackend
+        onSelectionToBeChanged: {
+            if (__dirty) {
+                lineEdit.backendValue.value = text
+            }
+            __dirty = false
+        }
+    }
+
     onEditingFinished: {
 
         if (hasToConvertColor)