Unmanage on "screen-changed" and "destroy-event".
Internal event filtering functions now private.
Cleaning up message (balloon) assembly code.
Added expiry of partial messages (balloons).
Messages (balloons) can now contain NULL bytes.

Changed the "message" signal to include an "n_message" parameter
indicating the length of the message. The encoding of balloon
messages is not well specified, so it is conceivable that messages
could contain NULL bytes.
Forgot to remove virtual methods from header file.