notify-bus.scm: Turn to singleton.

* src/notify-bus.scm: Turn to singleton.
  (notify-send): Improve.
* src/awgetd.scm (<awgetd>): Remove `notify-bus' slot.
  (initialize): Move creation of notify bus instance to `run-awgetd'
  procedure.
  (run-awgetd): Set default notify bus.
  (aworker-main-loop): Update.