TraitsView doesn't refresh when its view changes

`TraitsView` should `@observe()` its 'view' and 'model' attributes (atoms?). That way you can switch the view or model at runtime and have things work as expected. You can probably do this now with an enaml `Include` declaration, but I think that's asking too much.

Start by locating the TraitsView implementation and checking how its view and model attributes are declared and observed. Verify the existing behavior, then confirm that changing either attribute at runtime refreshes the displayed view or model as expected; the issue does not name a test file.

Written by the indexing model from the issue text.