BHV-4105: Update CHANGELOG and handle one-off joined panels case.

Issue
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If the second panel in `moon.Panels` is a joined panel, the branding image is displayed when returning to the first panel.

Fix
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This is kind of a one-off case: the `joinToPrev` property is examined along with the panel index before setting the `background-image` accordingly. Additionally, the API changes have been added to the CHANGELOG.

Enyo-DCO-1.1-Signed-off-by: Aaron Tam <aaron.tam@lge.com>