Signal which menu has updated in libdbusmenu-qt

Summary:
Then update appet, SNI, and appmenu appropriately which are only care
about the root menu.

This fixes some premature showing and re-evaluation.

Test Plan:
Ran statusnotifiertest and checked submenu worked
Ran applet and checked everything was still fine
Restarted kded and checked window decoration menu

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D4161