Generate DBusMenu Interface for libdbusmenuqt

Summary:
This not only makes the code cleaner but more importantly avoids plasma
making a blocking DBus call to a client in the QDBusInterface
constructor
which does a blocking call to Introspect

Test Plan: Ran my test app with the dynamic menu. Everything was still
fine

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D4172