fix issues with Qt application object in kwallet plugin

Use QGuiApplication instead of QCoreApplication to avoid runtime warning "Cannot use KWindowSystem without a QGuiApplication"
Provide argc and argv for QGuiApplication object as its documentation requires:
"The data referred to by argc and argv must stay valid for the entire lifetime of the QGuiApplication object. In addition, argc must be greater than zero and argv must contain at least one valid character string."

Testing Done:
* set kwallet as credential provider
* connected account with saved credentials
* disconnected credential provider externally
* created and connected new account and saved credentials
* restarted pidgin to let accounts autoconnect

Reviewed at https://reviews.imfreedom.org/r/2338/