Strings in UssdHandler class were made static
Error generated when DBus interface is not valid made more descriptive
USSD request will now be on a seperate thread

UssdHandler has been made a QThread & renamed as UssdThread.
Also, send button will also be disabled till the request completes &
the thread finishes.
ussd_handler.* changed to ussd_thread.*

This was required because ussd_handler was made a QThread.