Add icon to "Move to Desktop/Activity" task manager context menu item and fix capitalizations

Summary: {F7328150}

Test Plan: Open the context menu for a task manager item.

Reviewers: #plasma, #vdg, ngraham, davidedmundson

Reviewed By: #plasma, #vdg, ngraham, davidedmundson

Subscribers: ngraham, #vdg, plasma-devel, #plasma

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D23776