[System Tray] Listen to external collapse

Summary:
when the panel containment collapses all its applets,
forward the event right to the internal systray and close the popup

Test Plan:
pinned systray correctly closes when opening a panel controller
replaces D4842

Reviewers: Plasma, broulik

Reviewed By: Plasma, broulik

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D4865