think through widgets in central column, not just passages

[jtauber]

At the moment, the assumption is there is a passage in the middle (or nothing if no passage is being viewed) and all widgets are on the left or right. But it should be possible for certain other widgets to go in the middle if the overall "thing" being focused on is something other than a passage.

For example, it should be possible to have a collection, text group, or work as the main thing being focused on.

At the other end of the spectrum, it should be possible for a word to be the thing being focused on.

[jtauber]

[jtauber]

I think I'll start this by writing a main panel KWIC component