Make Appmenu work based on the presence of a visual representation

Summary:
drop the internal settings InApp,Menu, Decoration
but instead export the menu based on the presence of a dbus service called
org.kde.appmenuview, exported by either the menubar applet or the
kwin decoration button

let the decoration button and the globalmenubar work at the same time

not necessary anymore to have the setting in the kcm

Test Plan:
adding an appmenu applet causes apps started afterwards
to export their menubar, removing it causes new apps to
have their menubar internal again

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: davidedmundson, ngraham, mvourlakos, broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D9215