Simplify AppMenu menu management

Summary:
Instead of having a menu from the importer and our custom vertical menu
then syncing entries we can simply use the factory method in
DBusMenuImporter to create our custom one from the start.

This makes memory management easier as we only need to worry about the
importer which implicitly owns the menu.

Test Plan: Ran in-window menu, everything worked for me perfectly.

Reviewers: #plasma, broulik

Reviewed By: #plasma, broulik

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D4062