Adds new in-order traversal for MoveFocus and SwapPane actions.
Refactors the Pane methods to share a `NavigateDirection`
implementation.
Closes#10909
A large amount of the churn here is just renaming some of the things for
directional movement to reflect that it might not always be based on the
focused pane. `NextPane` and `PreviousPane` are the functions that
actually select the next/previous pane respectively and are the core
component of this PR.
VALIDATION
Created multiple panes on a tab, and tried both forward and backwards
movements with move-focus and swap-pane.
68294f863d