Use a distinct and more appropriate icon for Layers

Summary: BUG: 387424

Test Plan:
Tested in KDE Neon. Before, using Breeze icons:
{F5517757}

After, using Breeze icons:
{F5523390}

Before, using Oxygen icons:
{F5523394}

After, using Oxygen icons:
{F5523395}

Reviewers: aacid, #okular

Reviewed By: aacid, #okular

Subscribers: rkflx

Tags: #okular

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