Add button to toggle sidebar in fullscreen mode

Summary:
Previously, there was no way to toggle the sidebar in
fullscreen view mode except for the keyboard shortcut F4. This patch
adds a button to toggle the sidebar from the fullscreen toolbar.
BUG: 414694

Test Plan:
Tested the button in fullscreen (view) mode. Button works
as expected, and preserves existing behavior of all other sidebar
toggle mechanisms as well.

Reviewers: #vdg, #gwenview, ngraham

Subscribers: #gwenview

Tags: #gwenview

Differential Revision: https://phabricator.kde.org/D26230