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/textdocument.h

@@ -112,6 +112,7 @@ public:
     TextMarks marksAt(int line) const;
     void removeMark(TextMark *mark);
     void updateLayout() const;
+    void scheduleUpdateLayout() const;
     void updateMark(TextMark *mark);
     void moveMark(TextMark *mark, int previousLine);
     void removeMarkFromMarksCache(TextMark *mark);