Hide menu if the menu disappears off DBus

Summary:
This can happen without a focus change if focus is changed to a window
that doesn't claim the menu, for example the desktop.

It could also happen if an application does something weird, like
release
its service name.

BUG: 374995

Test Plan:
Ran "sleep 10; killall kate"
Then selected kate then the desktop
Kate remained showing for 10 seconds then closed

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D4177