Introduce the ability to configure stylesets, allowing customization of
aerc's look (color scheme, font weight, etc). Default styleset is
installed to /path/to/aerc/stylesets/default.
The following functionalities are added to configure aerc ui styles.
- Read stylesets from file with very basic fnmatch wildcard matching
- Add default styleset
- Support different stylesets as part of UiConfig allowing contextual
styles.
- Move widgets/ui elements to use the stylesets.
- Add configuration manual for the styleset
