D18358: [WIP] Add buttons to collapse and expand all elements

[Joao Oliveira]

joaonetto added a reviewer: VDG.

REPOSITORY
  R223 Okular

REVISION DETAIL
  https://phabricator.kde.org/D18358

To: joaonetto, #okular, #vdg
Cc: ngraham, okular-devel, tfella, darcyshen, aacid