Arrow key navigation is strange

[djdanlib]

Keyboard/controller arrow keys move focus strangely through the tree, at least according to normal conventions.

Left arrow on a leaf is normally expected to always go up to the leaf's containing branch. Currently, if you've selected the last fully-visible leaf under a branch containing several leaves, it will go down to the next one instead.

Left arrow on a branch is normally expected to collapse it, and right arrow is conversely expected to expand it. It's weird that those arrow keys instead navigate between the branch's expand/collapse button and the branch's label, when conceptually they are the same element.

Arrow key navigation is useful for controller, keyboard, and remote control interaction on devices that don't have touch or mouse input. Making this work better for that style of navigation would make this already good project even better.

Proposing a way to fix this is beyond my skills at the moment, sorry!