Update vector layer edits rollback method and app rollback actions

- Make optional whether edit buffer is deleted
- Returns optional functionality where rollback was similar to setting undo stack to index 0

src/app/qgisapp.cpp

@@ -5449,7 +5449,7 @@ void QgisApp::cancelEdits( QgsMapLayer *layer, bool leaveEditable, bool triggerR
     mSaveRollbackInProgress = true;
 
   mMapCanvas->freeze( true );
-  if ( !vlayer->rollBack() )
+  if ( !vlayer->rollBack( !leaveEditable ) )
   {
     mSaveRollbackInProgress = false;
     QMessageBox::information( 0,

src/core/qgsvectorlayer.cpp

@@ -3326,7 +3326,7 @@ const QStringList &QgsVectorLayer::commitErrors()
   return mCommitErrors;
 }
 
-bool QgsVectorLayer::rollBack()
+bool QgsVectorLayer::rollBack( bool deleteBuffer )
 {
   if ( !mEditBuffer )
   {
@@ -3344,8 +3344,12 @@ bool QgsVectorLayer::rollBack()
 
   updateFields();
 
-  delete mEditBuffer;
-  mEditBuffer = 0;
+  if ( deleteBuffer )
+  {
+    delete mEditBuffer;
+    mEditBuffer = 0;
+    undoStack()->clear();
+  }
   emit editingStopped();
 
   // invalidate the cache so the layer updates properly to show its original

src/core/qgsvectorlayer.h

@@ -713,8 +713,10 @@ class CORE_EXPORT QgsVectorLayer : public QgsMapLayer
     bool commitChanges();
     const QStringList &commitErrors();
 
-    /** Stop editing and discard the edits */
-    bool rollBack();
+    /** Stop editing and discard the edits
+     * @param deleteBuffer whether to delete editing buffer (added in 1.9)
+     */
+    bool rollBack( bool deleteBuffer = true );
 
     /**get edit type*/
     EditType editType( int idx );