In "Nodes only" mode, multiple topics not shown

[vpradeep07]

This is a repeat of #12. Since that issue was closed with no explanation, I'm re-raising the issue.

In my scenario:

• /talker publishes two topics: /TopicA & /TopicB
• /listener subscribes to the same two topics /TopicA & /TopicB

rqt_graph looks great when in "Nodes/Topics" mode:

However, when in "Nodes only" mode, only one topic is shown:

In my opinion there are three possible ways to resolve this:
Option 1: Do not display topic names when in "Nodes only" mode
Option 2: Display multiple graph edges; one edge for each topic.
Option 3: Display a single graph edge, and list all topics on that edge

Leaving the implementation as-is is confusing, since it provides only partial information.

[vpradeep07]

This was tested on a debian install on ROS Kinetic, Ubuntu 16.04
Package: ros-kinetic-rqt-graph
Version: 0.4.9-0xenial-20180830-080333-0800
Architecture: amd64

[vpradeep07]

Issue addressed in #21