Uses white-on-black for the popup if system is in dark mode.

Now supports different settings for each folder. Right-click on a folder to change its settings. Per-folder settings are not inherited by subfolders.

Added a "Nothing" sort which will leave the order untouched. This is useful in combination with the new per-folder settings.

Fixed a bug that, in some cases, prevented the sort from working since Firefox 59.

Fixes treatment of separators in Firefox 57.
Adjusts UI slightly.

Heeds separators and does not sort across them.
Auto-sorts folders when bookmarks are added, changed, removed, or moved between folders.