rqt_graph: Display topic/connection statistics along edges

[This depends on the TopicStatistics patches to ros_comm and ros_comm_msgs.
Also a small change to qt_gui_core is necessary.]

https://github.com/ros/ros_comm/pull/383

If TopicStatistics are available (/statistics) then show them along the edges.

Displayed are
* frequency, delay (based on the header.stamp) and traffic (in kb/s)
  as text
* delay again using the color of the edge is an indicator (green to red)
* traffic using the penwidth of the edge

Screenshot:

![stats_viewer](https://cloud.githubusercontent.com/assets/676900/2579279/776bbb50-b99f-11e3-9eb8-a5451d611c08.png)