Themes

Full theme support (#84). Currently supports:

- Themes bundled with LD
- Themes added by the user, through manual entry of data
- Themes added by the user, through linking to a remote JSON file

Remote themes are automatically updated once a day.

Details on creating themes can be found in THEMES.md

Test themes ("Red background" & "Theme 2") are currently bundled. They should be replaced by actual themes before release.
Figured that since it was in lib, it was used in other places than settings, so I didn't want to touch it.