Allow users to disable (C++) syntax error highlighting

Qt Creator 2.4.0 does not appear to allow users to stop it from putting red and yellow squiggles under lines containing syntax errors or lines containing C++11 features that the syntax checker does not understand. Even if the syntax checker supported all C++11 features and never made mistakes, users should still be able to make it fail silently, if they'd prefer to never see the distracting squiggles.