This brings back global menu support in KWin.
The DBusMenu infrastructure is different that we just read the DBus service name and
menu object path from the windows rather than passing around window IDs on DBus which
won't work on Wayland.
Differential Revision: https://phabricator.kde.org/D3089
The decoration button configuration interface is merged into the QML
part using two list views (left buttons, right buttons) and a grid
view for all the available buttons.