[2012-06-01] Expand tree nodes when navigating in pages list view

[silverstripe-issues]

created by: @chillu (ischommer)
created at: 2012-06-01
original ticket: http://open.silverstripe.org/ticket/7420

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When navigating into deeper levels of the page list view, the tree view stays the same.
Expand tree nodes as required (which might necessitate loading more nodes via AJAX).
Also applies to the assets view, ideally implemented in a generic way which applies to both.
Related to #421

We could mark the parent node from the list view as selected in the tree as well,
but that'll get weird once you switch back to using the tree. E.g. you might think
that batch actions only apply to the selected node (but they're reliant on the checkbox selections).
There's also no way to "unselect" a selected node in the tree.
Note: When I say "selected node", I mean the light blue background styling, not the checkbox state.

[tractorcow]

Related to #448