Second tentative fix to #8159: Undo doesn't restore environment depth correctly

In this version, the idea is to record undo at the place where the document is modified, which is definitely cleaner.

1/ in Buffer::updateBuffer, add a recordUndo, with the caveat that a
   const_cast has to be used

2/ in GuiApplication::dispatch, add an extra undo group that
   encompasses the updateBuffer call. Some other undo groups may be
   redundant now, but it is not a problem since they do not cost
   anything.

src/Buffer.cpp

@@ -4413,7 +4413,17 @@ void Buffer::updateBuffer(ParIterator & parit, UpdateType utype) const
 	pit_type const lastpit = parit.lastpit();
 	for ( ; parit.pit() <= lastpit ; ++parit.pit()) {
 		// reduce depth if necessary
-		parit->params().depth(min(parit->params().depth(), maxdepth));
+		if (parit->params().depth() > maxdepth) {
+			/** FIXME: this function is const, but
+			 * nevertheless it modifies the buffer. To be
+			 * cleaner, one should modify the buffer in
+			 * another function, which is actually
+			 * non-const. This would however be costly in
+			 * terms of code duplication.
+			 */
+			const_cast<Buffer *>(this)->undo().recordUndo(parit);
+			parit->params().depth(maxdepth);
+		}
 		maxdepth = parit->getMaxDepthAfter();
 
 		if (utype == OutputUpdate) {

src/frontends/qt4/GuiApplication.cpp

@@ -1132,8 +1132,13 @@ static docstring makeDispatchMessage(docstring const & msg,
 
 void GuiApplication::dispatch(FuncRequest const & cmd)
 {
-	if (current_view_ && current_view_->currentBufferView())
+	Buffer * buffer = 0;
+	if (current_view_ && current_view_->currentBufferView()) {
 		current_view_->currentBufferView()->cursor().saveBeforeDispatchPosXY();
+		buffer = &current_view_->currentBufferView()->buffer();
+		if (buffer)
+			buffer->undo().beginUndoGroup();
+	}
 
 	DispatchResult dr;
 	// redraw the screen at the end (first of the two drawing steps).
@@ -1141,6 +1146,10 @@ void GuiApplication::dispatch(FuncRequest const & cmd)
 	dr.screenUpdate(Update::FitCursor);
 	dispatch(cmd, dr);
 	updateCurrentView(cmd, dr);
+
+	// the buffer may have been closed by one action
+	if (theBufferList().isLoaded(buffer))
+		buffer->undo().endUndoGroup();
 }