QmlOutline: Speed up updates

Guard the incremental updates with beginResetModel/endResetModel

This gives better overall performance then passing the incremental
updates to the views.

Tested-by: Lasse Holmstedt

src/plugins/qmljseditor/qmloutlinemodel.cpp

@@ -367,6 +367,10 @@ void QmlOutlineModel::update(const SemanticInfo &semanticInfo)
     m_treePos.append(0);
     m_currentItem = invisibleRootItem();
 
+    // resetModel() actually gives better average performance
+    // then the incremental updates.
+    beginResetModel();
+
     m_typeToIcon.clear();
     m_itemToNode.clear();
     m_itemToIdNode.clear();
@@ -376,6 +380,8 @@ void QmlOutlineModel::update(const SemanticInfo &semanticInfo)
     QmlOutlineModelSync syncModel(this);
     syncModel(m_semanticInfo.document);
 
+    endResetModel();
+
     emit updated();
 }