Make methods to introspect the viewer (and subset?) names in an application

[eteq]

This issue was initially prompted by #159 (comment) but is a bit broader so I'm making it a separate issue.

Basically, the issue here is that all of the convenient data accessors in jdaviz.Application depend on knowing the name of the viewers in the configuration file. This is not desirable because it's not really easily visible to the user, and we do want users to be able to use these APIs for accessing data. So there needs to be a way for a user to ask "what are all the viewers in this app".

I think the most natural way to do this is to simply add a get_viewer_names method to Application? It would then just return a list of all the relevant names that go into the various get_<something>_from_viewer references.

At the same time it probably makes sense to add a similar set of methods for subset names. This is less critical since the user can by-eye see them in the UI, but it's useful for writing code that does something over all of the subsets (and would be applied in combination with the dicts that come out of #159 and #161).

[pllim]

Ops... Didn't see this until after I created #643. Since that one has a JIRA ticket attached to it, I am closing this as duplicate. Sorry!