Bookmark bar uses "Bookmark bar" subfolder

* Allows having a lot of bookmarks in the menu, and only the most often
used ones in the bar.
* If the "Bookmark bar" folder doesn't exist, the old behavior (using
the main Bookmarks folder for the bar as well as the menu) is used.
* If none of these exists, the bookmark bar will not be available (just
like the bookmark menu)

Fixes #11092.

We may want to update the default set of bookmark to include some links
in the bar.