Make QgsVectorLayerExporter::flushBuffer public

Sometimes it's necessary to manually call this, instead of
deleting the exporter and letting it be called automatically
There's a chance that features in the buffer will cause errors
while being exported, and if the only way to flush the buffer
is to delete the exporter then there's no way to detect or
retrieve these errors.

python/core/qgsvectorlayerexporter.sip

@@ -124,6 +124,13 @@ class QgsVectorLayerExporter : QgsFeatureSink
  Finalizes the export and closes the new created layer.
 %End
 
+    bool flushBuffer();
+%Docstring
+ Flush the buffer writing the features to the new layer.
+.. versionadded:: 3.0
+ :rtype: bool
+%End
+
   private:
     QgsVectorLayerExporter( const QgsVectorLayerExporter &rh );
 };

src/core/qgsvectorlayerexporter.h

@@ -141,10 +141,14 @@ class CORE_EXPORT QgsVectorLayerExporter : public QgsFeatureSink
      */
     ~QgsVectorLayerExporter();
 
-  private:
-    //! Flush the buffer writing the features to the new layer
+    /**
+     * Flush the buffer writing the features to the new layer.
+     * \since QGIS 3.0
+     */
     bool flushBuffer();
 
+  private:
+
     //! Create index
     bool createSpatialIndex();