Review request: AppMenu support for KDE

[Lionel Chauvin]

The code that bring support of AppMenu to KDE needs to be reviewed before it entered in KDE main module:
https://projects.kde.org/projects/kdereview/kded-appmenu/repository

It contains a KDED module and a library.
The KDED module exports applications menu through dbus.
The library exposes the functionalities of the module so it is not needed to deal with KDED stuff.

This support is required by the menu button in the oxygen decoration:
https://git.reviewboard.kde.org/r/104344/

It can be tested using an adapted version of the plasmoid menubar:
https://code.launchpad.net/~megabigbug/plasma-widget-menubar/appmenu-kde