Provide scripts with an env var to find qdbus

While the qdbus tool has been a RUNTIME dependency of Konversation since 2008, most distros didn't add a requirement. Given there's no standard way to advertise runtime deps, I've tried to make this into a noisy build dep.

Unfortunately qdbus doesn't have a consistent name nor location. This attempts to find it by asking Qt where the binaries probably got installed and then tries a few possible names.

A script run from inside Konversation now expects to receive an environment variable "KONVERSATION_DBUS_BIN" that is the full pathname of the actual program. This is set by the script launcher if it isn't present in the environment on launch.