TextEditor: schedule update document layout

When generating a lot of text marks we do not need to update the
document for every mark and every property that changes.

Change-Id: Ibba80138f6ebcbbe30a4f741979df69ad62608c1
Reviewed-by: hjk <hjk@qt.io>

src/plugins/texteditor/textdocumentlayout.h

@@ -237,6 +237,11 @@ public:
     void documentReloaded(TextMarks marks, TextDocument *baseextDocument);
     void updateMarksLineNumber();
     void updateMarksBlock(const QTextBlock &block);
+    void scheduleUpdate();
+    void requestUpdateNow();
+
+private:
+    bool m_updateScheduled = false;
 
 signals:
     void updateExtraArea();