places-sidebar:  Add dedicated bookmarks section.

New behavior:
- New user - xdg folders display under home/desktop, as before
- Upgrading user - all bookmarks are unchanged, still display in current position.
- Adding new bookmarks via menus - new items are placed under a Bookmarks category.
  They can be dragged to the My Computer section if desired.
- Performing any sort of drag operation (either a new drag from outside the sidebar
  or re-arranging bookmarks, will temporarily display the Bookmarks heading, to
  reveal the new section.  If the user ends the drag without dropping anything in this
  new section, the heading disappears again.
- Bookmarks can be dragged around between sections without restriction - the bookmark
  section will only show if there are items within it.