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Outliner: Left and right walk navigation

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