Only activate kded Appmenu signals if menus are in the window decoration

Summary:
The Appmenu applet doesn't use the AppMenu module, and it interferes
with the applet.

Test Plan:
Set menus to in decoration
Pressed alt+f, it appeared
Changed it to be in the applet
pressed alt+f, nothing happened

Reviewers: #plasma, broulik

Reviewed By: #plasma, broulik

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D4833