Background image on sections and blocks

TODO:
- refactor display processing
- Form to control which media bundle is available and with which field.
- UI Patterns attributes plugin source

Also need to be able to handle background image on block contents by adding a media field on the block type and so view displays could use it with tokens.

Discussed with @pdureau,

"Need to be able to handle background image on block contents by adding a media field on the block type and so view displays could use it with tokens."

Need Display Builder, or another source doing complicated stuff with tokens for that need.

Let's discuss if it is needed and if current state could be merged.