Optimization of isOfType method.

According to profiling the isOfType method used a lot of extra time
converting the classname attribute from const char to QString and back
to const char. This task fixes the attribute to be of type const char
to begin with, reducing the CPU usage of isOfType significantly.

Change-Id: Id4879c376701d2baf4fa28ca39e9fcc1c3c0b5d6
Reviewed-by: Miikka Heikkinen <miikka.heikkinen@theqtcompany.com>