notify-send: Support for replacing an existing notification

Support printing and overriding notification ID, so that it's possible
to replace a notification that is currently showing with an updated
content.

Based on patch as provided by Paul Collins in the following bug
report:
  https://bugs.launchpad.net/ubuntu/+source/libnotify/+bug/257135/comments/8



Co-authored-by: Marco Trevisan (Treviño) <mail@3v1n0.net>

Fixes #19