Connect aboutToHide signal from QMenu to relevant libdbusmenu-qt slot

Summary:
This sends the close event to the client which then sends aboutToHide to
the QMenu.

Code was all there, just not hooked up

BUG: 375053

Test Plan: Added qDebug to the test app inside libdbusmenuqt

Reviewers: #plasma, markg, broulik

Reviewed By: #plasma, markg, broulik

Subscribers: broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D5192