TreeView control for Landmarks

[matatk]

This will lay the foundation for providing headings in future. It will also mean that the tree has one tab stop and thus makes it possible to include help and options links in the pop-up directly.

It will allow all those UI changes; it also seems like a good opportunity to start moving to considering the landmark regions (and other things) found by the content script as a tree structure rather than a list. Landmarks can be inside other landmarks, and headings can be inside landmarks too.

To ensure that the results are correct, the tree should be flattened into a list during development, so that the code can be changed without changing the tests, then the tests can be updated once the code is known to be working.