[Qt] Replace status bar unit icon with actual images

The icon introduced by #4302 is too big in my opinion on the linuxes and windows I tested.
![gubatron1](https://cloud.githubusercontent.com/assets/2814559/3486919/52d68c98-0459-11e4-999e-a60f74785784.png)
![gubatron2](https://cloud.githubusercontent.com/assets/2814559/3486921/5635f518-0459-11e4-8f92-cce99f965137.png)
![gubatron3](https://cloud.githubusercontent.com/assets/2814559/3486922/58e65316-0459-11e4-95e9-0499a02ad711.png)

Reducing the font-size to 8pt comes close, but then its still vertically misaligned a few pixels depending on the desktop.

So this pull uses actual images, which look the same over all platforms and desktops.
![gubatron9](https://cloud.githubusercontent.com/assets/2814559/3486933/fe34904e-0459-11e4-819d-a9a010fa1747.png)