Add getBehavior and include continuous behavior in isContinuous

[jamesmisson]

Some changes necessary for UV issue 1503 UniversalViewer/universalviewer#1503

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[demiankatz]

Looks good to me! Obviously the problems discussed elsewhere regarding multi-valued behaviors and viewing hints apply here as well, but that's a bigger problem, and following existing convention should be good enough to get us off the ground for now.

In the long run, I wonder if we need to add getBehaviors and getViewingHints methods, and then start replacing the legacy singular versions with the new plural ones (complete with "array contains" checks instead of straight string comparisons).

It might also be a good idea to add more tests here -- it does appear that some testing infrastructure exists -- but maybe that's a separate project too; I'll leave it to your discretion.

In any case, I'm approving this now, and you can merge whenever you feel ready.