234 - Add Groups to Bookmarks 

Issue #234

- From the bookmark editing view, set the group for that bookmark.
- Inside the group list groups can be added, renamed, and deleted.
- Bookmarks can not be moved between groups by dragging.
- Editing in the bookmarks list allows deletion of groups as well as
  bookmarks.
- Deleting a group doesn't delete the bookmarks, the bookmarks are
  removed from the group before deleting.
- Groups can be collapsed and expanded in the bookmarks list.