Mark folders as categories

[flaksp]

Add ability to mark folders as categories in options.

In this example, "Work" is a folder turned into a category:

Implementation requirements:

• Only root-level folders may be presented as categories. Root-level folders are folders placed in the root of "Bookmarks Bar" and "Other Bookmarks" folders.
• If folder turned into a category, it should be hidden from "Bookmarks Bar" and "Other Bookmarks" folders.
• On the Options page there should be a list of root-level folders with checkboxes. It should be possible to mark and unmark folders, and change order of categories.