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:

Without it:

Reviewers: Okular, VDG, ngraham

Subscribers: ndavis, aacid, ngraham, okular-devel

Tags: Okular

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