Use ellipsis on "Add Source" menu item because it opens a dialog
Summary: UI elements that open dialogs should use ellipses to communicate this. The "Add Source" menu item does, so it needs them.
Test Plan:
Tested in KDE Neon:
Reviewers: apol, Discover Software Store
Reviewed By: apol, Discover Software Store
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D9861