Web: Move tree and tree table to browsable in sticky browse mode only
Web: Move tree and tree table to browsable in sticky browse mode only

Depending on the user's settings and navigation methods, trees and
tree tables do not automatically cause Orca to switch to focus mode.
Due to some recent changes and fixes to make badly-authored ARIA
more readable, trees should now be relatively accessible in browse
mode. However, this improvement in Orca can make web app enhanced
functionality less discoverable which in turn can make those authors
unhappy.

In order to balance everyone's needs and preferences, we will not
descend trees and tree tables unless we are in sticky browse mode.
This should accomplish the following:

* Trees that are not of interest can be quickly skipped over.
* Users are more likely to remember to switch to focus mode to
  use the author-provided navigation in order to descend the tree.
* If that author-provided navigation is lacking, sticky browse mode
  can still be used to read and interact with the content.

GNOME/orca,GNOME/orca,GNOME/orca,GNOME/orca