Add icons for the Trim Margins tools

Summary:
New icons were added in D18552.

BUG: 397768
FIXED-IN: 19.04.0

Test Plan: {F6569231, size=full}

Reviewers: #okular, #vdg, ndavis

Reviewed By: #vdg, ndavis

Subscribers: ndavis, trickyricky26, aacid, okular-devel

Tags: #okular

Differential Revision: https://phabricator.kde.org/D15193