Add a constructor of UndoGroupHelper that uses a CursorData parameter

This aloows to se the the `before' cursor of the undo group.

Not used for now.

src/Undo.cpp

@@ -704,6 +704,12 @@ UndoGroupHelper::UndoGroupHelper(Buffer * buf) : d(new UndoGroupHelper::Impl)
 }
 
 
+UndoGroupHelper::UndoGroupHelper(CursorData & cur) : d(new UndoGroupHelper::Impl)
+{
+	resetBuffer(cur);
+}
+
+
 UndoGroupHelper::~UndoGroupHelper()
 {
 	for (Buffer * buf : d->buffers_)
@@ -712,6 +718,7 @@ UndoGroupHelper::~UndoGroupHelper()
 	delete d;
 }
 
+
 void UndoGroupHelper::resetBuffer(Buffer * buf)
 {
 	if (buf && d->buffers_.count(buf) == 0) {
@@ -721,4 +728,13 @@ void UndoGroupHelper::resetBuffer(Buffer * buf)
 }
 
 
+void UndoGroupHelper::resetBuffer(CursorData & cur)
+{
+	if (!cur.empty() && d->buffers_.count(cur.buffer()) == 0) {
+		d->buffers_.insert(cur.buffer());
+		cur.buffer()->undo().beginUndoGroup(cur);
+	}
+}
+
+
 } // namespace lyx

src/Undo.h

@@ -140,12 +140,18 @@ class UndoGroupHelper {
 public:
 	// Begin a new undo group for buffer \c buf.
 	UndoGroupHelper(Buffer * buf);
+	// Begin an undo group for the buffer of \c cur with the the
+	// `before' cursor set to \c cur.
+	UndoGroupHelper(CursorData & cur);
 	// End all active undo groups.
 	~UndoGroupHelper();
 
 	// Begin if needed an undo group for buffer \c buf.
 	void resetBuffer(Buffer * buf);
 
+	// Begin if needed an undo group for the buffer of \c cur with the
+	// the `before' cursor set to \c cur.
+	void resetBuffer(CursorData & cur);
 private:
 	class Impl;
 	Impl * const d;