Improves power monitor UI

Improves power monitor sensor list UI so that sensors with distinct powernets can be seen at a glance. 
Fixes #7449

(kind of outdated screenshot)
![screenshot 2014-12-26 16 48 00](https://cloud.githubusercontent.com/assets/242428/5559538/0af6c0f6-8d1f-11e4-975f-666b5a87cb5b.png)