Add EnumClassWithStrings macro.

This enriches EnumClass with ctor(QString) and operator QString().

Ctor asserts that string passed as argument represents a valid label in
enum.

Example usage:

--------------------------------------------------------------

EnumClassWithStrings(Test, quint8, label_A, label_B, label_C);

int main()
{
	Test a("label_A");
	QString s(a);
	QTextStream stream(stdout);
	stream << s << "\n";
	return 0;
}
--------------------------------------------------------------