Add background image and graphic options

[swashbuck]

Subject of the enhancement

The Language Picker default screen is very simplistic. It would add visual interest if we could have the same background image and graphic (to add a logo) as with Box Menu. Currently, adding a background graphic requires theme customizations and the image cannot be changed via JSON without customizing the template.

https://github.com/adaptlearning/adapt-contrib-boxMenu/blob/09f9db83f12884a1956143252b1d3bf8d0319486/example.json#L2-L42