UI: Refactor panel alignment function

This code had grown overly complicated and hard to understand.
The improvements in this commit:
 - Avoid allocating a duplicate of every active panel.
 - Instead of complicated logic to calculate each panel's offset,
   just keep track of the current offset while iterating through.
 - More readable code structure, better comments.

Note that calcuting the X offset here is a relic from pre-2.5 when
panels could be aligned horizontally. I kept this in, but it would
be reasonable to remove it in the future.

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