oceans: fix name, fallback colour, and build

I wasn't sure if these couple of issues were known and/or intentional but I figured I would setup a merge request anyway.

The XML file for the "Oceans" background seemed to be left out of the meson.build file in backgrounds, so it is not included in builds. Further, its definition in the XML file seemed to be the same as the "Fields" for name and fallback colour. So, include the XML file in meson.build and update oceans.xml.in to have the correct name and a fallback colour selected from the image.

I also wasn't sure what specific criteria or guidelines existed for selecting a fallback colour, so I chose #6aadd5, a blue in the image that seems to come close to the "average" colour of the image.