Add buttons to collapse and expand all elements

Summary:
Adds two buttons on the bottom right to expand and collapse all elements in annotations
BUG: 377024
T8553

With latest breeze-icons:
{F6687517}

Without it:

{F6687519}

{F6687518}

Reviewers: #okular, #vdg, ngraham

Subscribers: ndavis, aacid, ngraham, okular-devel

Tags: #okular

Differential Revision: https://phabricator.kde.org/D18358