Rename Display topic accessors and add documentation

Change "setROSTopic()" to "setTopic()" and "getROSTopicTypes()" to
"getTopicTypes()".  There is no non-ROS message support in rviz, so these
changes result in shorter and still unambiguous method names.
Use a tree with combo boxes for selection by topic

In "Add display by topic dialog" topics are now displayed hierarchically.
If a topic is visualizable (i.e., at least one plugin it says it can use
it) all plugins that can visualize that topic are listed as children.  If
a plugin can view topics that are children of the visualizable topic, they
are displayed as dropdown options in a combo box of the plugin.