Use monochrome symbolic icon for edit as new action

Summary: All other icons in the context menu are monochrome, so this one should too to be consistent

Test Plan: {F7856439}

Reviewers: #kde_pim_kmail, vkrause

Reviewed By: vkrause

Subscribers: vkrause, kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D26380