[QT] Add network traffic graph to debug window

This change adds a Network Traffic tab to the debug window, containing total bytes in/out labels (formatted as KB, MB, GB) and a graph that shows sampled bytes per second in and out. The total time period covered by the graph is adjustable with a slider.
![snapshot2](https://f.cloud.github.com/assets/214252/1010300/24a85c1c-0b48-11e3-92c6-0ca5c0dffd3a.png)