Utils: add convenience function to ChangeSet

Creating a QTextCursor just for the ChangeSet gives no benefit for the
calling code, but reduces the readability.

Change-Id: I34acb6083b6f7ab54fce042e29cd6e80498338ef
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/libs/utils/changeset.h

@@ -10,6 +10,7 @@
 
 QT_BEGIN_NAMESPACE
 class QTextCursor;
+class QTextDocument;
 QT_END_NAMESPACE
 
 namespace Utils {
@@ -76,6 +77,7 @@ public:
 
     void apply(QString *s);
     void apply(QTextCursor *textCursor);
+    void apply(QTextDocument *document);
 
 private:
     // length-based API.