Refactor backgrounds

[in1tiate]

Background formatting sucks. We're still supporting legacy background names that can be updated with a literal CTRL+F find replace in the filenames. Design.ini is both staggeringly underutilised and underpowered.

New background format would require:

• Functional pos dropdown decoupled from all the legacy nonsense. Define your positions or die, no more autodetection.
• Arbitrary variable storage per-position. Design.ini can accomplish this.
• Separation of assets from data storage. Having a text file in a folder full of PNGs is messy.