* Use symbolic icons instead of labels that require as less
  space as the text. It is important to keep the buttons small
  to facilitate the resize of the sidebar (e.g. make it as
  narrow as one column of thumbnails)
* As the bookmarks button are now closer to the switcher, we
  mimic the same style of linked buttons, although we do not
  expand them to the sidebar to avoid clutter it.

See #947.