Use a distinct and more appropriate icon for Layers

Summary: BUG: 387424

Test Plan:
Tested in KDE Neon. Before, using Breeze icons:

After, using Breeze icons:

Before, using Oxygen icons:

After, using Oxygen icons:

Reviewers: aacid, Okular

Reviewed By: aacid, Okular

Subscribers: rkflx

Tags: Okular

Differential Revision: https://phabricator.kde.org/D9043