- Move theme installation to cli entry point (omf.fish)
- Generalize XDG_CONFIG_HOME code in `omf.xdg.config_home`
- Generalize prompt conflicts check in `omf.checks.fish_prompt`
  * Migrate `omf.doctor` to use it

+ Change theme activation steps
  - Create user function path if not existent
  - Check for conflicts in fish_prompt, aborts if found
  - Remove current theme from autoloading paths
  - Add target theme to autoloading paths
  - Link target theme's fish_prompt to user's
  - Persist selected theme to $OMF_CONFIG/theme

This approach brings two major advantages:

- Theme prompt now shows accordingly in fish_config web interface
- Faster theme changes, reducing it from seconds to some milis
- Avoids calling refresh, which makes users much more happy