Add icons for context and sidebar menu

Summary:
As requested by @rkflx, menu icons have been added for the context and sidebar menus, continuing the work completed in D10503.

Operations Sidebar Before:

Operations Sidebar After:

Context Before:

Context After:

Trash Context Before:

Trash Context After:

Test Plan: Execute Gwenview, perform moves/copies/links/redeye actions. Verify icon resolution and appropriateness.

Reviewers: Gwenview, ngraham, rkflx

Reviewed By: Gwenview, ngraham, rkflx

Subscribers: muhlenpfordt, ngraham, rkflx

Tags: Gwenview

Differential Revision: https://phabricator.kde.org/D10726