WebContent+UI: User configuration should be read from $XDG_CONFIG_DIR or similar

[ADKaster]

Currently we drop:

• Browser bookmarks JSON (not currently used by Ladybird, but are used by SerenityOS Browser)
• Auto-play allowlist config
• Content Filter denylist config

All in share/Lagom/ladybird. When installed from a distribution package, these config files will be read-only by a regular user.

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• [ ] Read per-user configuration files. We should copy these shipped default configs into the user's config directory on....

  • first launch?
  • any time or if they are not present
  • only if the user edits them through some custom UI (about:config?)
  • controlled by some flag that distros set if they want the default configs copied or not?

• [ ] Reconcile user config files with default config files

  • merge user config directories with the default configs?
  • override default configs with user configs?

• [ ] Import and export configuration files

  • [ ] provide a way to export user settings and preferences
  • [ ] provide a way to import user settings and preferences (from other browsers? or just ladybird's format?)

[OfficialKris]

IMO the Zed project handles this well. For Linux, they do not add any configs to ~/.config until the user specifically wants to override the defaults. Only a single file is used for config overrides. All other state data (extensions, project history) is in ~/.local/share.

I think it would be good to have something similar for ladybird.