Automatically close Clipboard applet after clearing history or deleting the last element

Summary:
It would be correct to close the Clipboard applet after clearing the history or deleting the last element, since there is nothing more to do with the empty Clipboard applet (similar to how the Notification applet closes after the last notification is removed).

BUG: 421345
FIXED-IN: 5.20

Reviewers: ngraham, #plasma, broulik

Reviewed By: ngraham

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D29652