Fixes #182 (Soda Dark 3, icons AND arrows)

* Sidebar tree with row_padding of [0, 3]
  Removes extra padding

* Sidebar group closed with content_margin of [0, 8]
  Removes arrows (and leaves just folders)

* Sidebar file icons with content_margin of [0, 8]
  Removes extra padding in files inside folders
  (making them look as if they were a level deeper than they are)