BHV-2871: Add support for branding image to Panels.

Issue
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We need a way to specify a branding image to be displayed in the lower left of `Panels`.

Fix
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A new property `brandingSrc` has been added to `Panels`. Additionally, some static widths (i.e. breadcrumb width) were pulled out into common variables, and opacity was changed to rgba so that the background image would not have opacity applied.

Enyo-DCO-1.1-Signed-off-by: Aaron Tam <aaron.tam@lge.com>