BHV-2240: Added support for moon.Header to use multiple background images and positions

* Additional bonus, backgroundSrc may now be set to `null` to remove the property and allow CSS to be written from a stylesheet.
* You may even set backgroundPosition to initial or inherit, and those values will be respected instead of assuming you want `background-position: inherit right;`

Enyo-DCO-1.1-Signed-off-by: Blake Stephens <blake.stephens@lge.com>