qt: pass char * to go functions from cpp helpers

instead of converting QString to GoString struct which is error prone, this changes SetTransport and SetLable in backend.go to receive a char* as arg. which is then converted to Go string using the C.GoString helper

this fixes a bug on macOS where changing preference options did not have any effect as the conversion of string was not working and empty values were passed to Go functions

fixes #845 (closed) and potentially #860 (closed)