Update the detection of the arguments in Qt's invokeMethod.

invokeMethod can invoke functions by name (string) rather than a
direct function call (i.e. reflection). The old code wasn't
correctly parsing out the argument which contained the name
of the function to call.

This resulted in that function being reported as unused when it is.
Tidy up the code for cppcheck style.
Merge pull request #263 from trulabs/master

Fix the detection of methods called from Qt's invokeMethod