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:
{F5646630}

Reviewers: apol, #discover_software_store

Reviewed By: apol, #discover_software_store

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D9861