Improve handling of disabled parser and re-enabling parser

Instead of always parsing again when re-enabling parser do it only
if there are changes of the current project.
If the current project has not changed since last parsing just rely
on the cached information.

Change-Id: I7681318132f37172730648604ddbecf1cd37d177
Reviewed-by: Oliver Wolff <oliver.wolff@theqtcompany.com>
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>

plugins/autotest/testcodeparser.h

@@ -113,6 +113,7 @@ private:
     bool m_pendingUpdate;
     bool m_fullUpdatePostPoned;
     bool m_partialUpdatePostPoned;
+    bool m_dirty;
     QSet<QString> m_postPonedFiles;
     State m_parserState;
 };