[style] speed up importXML() by using SQL transaction

Gains are significant, importing 100 symbols would take 2.86s,
but takes only 0.18s when using transaction.

src/core/symbology-ng/qgsstyle.cpp

@@ -1555,6 +1555,11 @@ bool QgsStyle::importXml( const QString& filename )
   QDomElement symbolsElement = docEl.firstChildElement( QStringLiteral( "symbols" ) );
   QDomElement e = symbolsElement.firstChildElement();
 
+  // gain speed by re-grouping the INSERT statements in a transaction
+  char* query;
+  query = sqlite3_mprintf( "BEGIN TRANSACTION;" );
+  runEmptyQuery( query );
+
   if ( version == STYLE_CURRENT_VERSION )
   {
     // For the new style, load symbols individualy
@@ -1639,6 +1644,9 @@ bool QgsStyle::importXml( const QString& filename )
     e = e.nextSiblingElement();
   }
 
+  query = sqlite3_mprintf( "COMMIT TRANSACTION;" );
+  runEmptyQuery( query );
+
   mFileName = filename;
   return true;
 }