[css-backgrounds][css-om] Move color to the beginning of <final-bg-layer> on serialization

In the <final-bg-layer> of the 'background' shorthand the
<background-color> definition is at the end of the definition.[1]

Some Firebug users brought up, that they'd prefer to have the color at the
beginning of the definition when the value is serialized.[2] Therefore I
created a Mozilla report a while back[3], though I believe this would
better be solved in the spec.

Sebastian

[1] http://www.w3.org/TR/css3-background/#ltfinal-bg-layergt
[2] https://code.google.com/p/fbug/issues/detail?id=4180
[3] https://bugzilla.mozilla.org/show_bug.cgi?id=743392