notifier: only show applet when necessary
Only show the notifier applet when there are interesting things to notify about:
- update notifications
- pending reboot
- and ongoing updates.
If we think that this completely fixes 413053, we can change CCBUG to BUG. Right now I am only marking it as CCBUG since the process still always runs in the backgroun. But from a UX point of view maybe we can consider the problem solved.
Edited by Yifan Zhu