Add KConfigSkeleton for per-backend options.
Move TextDocumentSettings{,Skeleton} into separate module.
Drop deprecated TODO's from font-selector-for-plain-text-formats
Initialize TextDocumentSettingsSkeleton in construtor.

To read current from from config, and load with it on startup
Codestyle: add spaces after/before brackets
Read font from settings skeleton instead of settings.

Settings is not updated on startup.