Allow Qul::Signal to take std::string as parameter

Allow Qul::Signal to take std::string as parameter, the issue is due to when trying to write a slot for a std::string signal, you currently get a Slot<void(Qul::Private::String)>, and the constructor call fails when passing the signal with the different argument type.

As a workaround, it compiles when defined as
Qul::Signal<void(Qul::Private::String str)>