Factorize waiting for notifications

On #8685 (closed) we updated these things for Jessie, and standardized them a bit, and as a result we are duplicating the same calls (pgrep -u "${LIVE_USERNAME}" '^ibus-daemon') all over the place. We should clean this up.

Implementation ideas:

move the existing code to is_notifications_ready() and wait_until_notifications_are_ready() in our gnome.sh shell library
introduce a systemd target and depend on / wait for it; and/or use systemd/D-Bus integration to require the notifications D-Bus service to be ready, and IIRC systemd can queue up messages until GNOME Shell is ready to receive the messages

Related issues

Related to #8685 (closed)

Original created by @intrigeri on 10657 (Redmine)

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