Modify Bean Tree and Detail views to allow independent scrolling

See https://github.com/hawtio/hawtio-online/issues/593 for details.

Transferred from https://github.com/hawtio/hawtio-online/issues/595.

### Summary

The Bean Tree and Detail views are both drawn inside the same scrollable container meaning that when scrolling down the former, the latter is also scrolled. Add a scrollbar to the Bean Tree that works independently of the Detail View.