cache parsed pri & prf files

no point in re-parsing them over and over during the scan of a bigger
project ...

shaves off another 20% of the loading time of Qt 4.6.

src/plugins/qt4projectmanager/qtversionmanager.cpp

@@ -1207,6 +1207,7 @@ void QtVersion::updateToolChainAndMkspec() const
 
     ProFileOption option;
     option.properties = versionInfo();
+    option.cache = ProFileCacheManager::instance()->cache();
     ProFileReader *reader = new ProFileReader(&option);
     reader->setCumulative(false);
     reader->setParsePreAndPostFiles(false);