Previously the left and right arrow keys would close and open the active
tree element, but a subsequent key press would not select up or
down the tree as is common in tree-based interfaces.
Walking left and right now does a selection action after opening or closing
the active tree item. For example, a right key press on a closed element
will open it's subtree, and an additional right key press will select
the first child element.
A left key press anywhere in a subtree will first close the active
element if it's subtree is expanded. Walking left again will select the
parent element.
Part of T77408
Differential Revision: https://developer.blender.org/D8650
70151e41dc