Container - advanced backgrounds support for specifying multiple image backgrounds

[eastriverstudio]

At the moment we can use advanced backgrounds for changing image per device or it is possible to have two image backgrounds by using different target type. However the css background property supports specifying multiple backgrounds in a comma delimited format.

As we have the advanced backgrounds feature in GBP already and it is possible to set more than one image using the UI it seems sensible to suggest that we could therefore allow mutiple backgrounds as per the shorthand css format.

So e.g if I specify more than one background image for the same device type GB would be smart enough to add all those to the same background rule and I can use the existing UI to position them individually etc.