Forward declare QtDBus classes

Summary: Forward declare QtDBus classes as we do not want to publically link to the library

Reviewers: O3 libqapt & debconf-kde, yurchor, sitter

Reviewed By: O3 libqapt & debconf-kde, yurchor, sitter

Subscribers: vanini, yurchor, sitter

Differential Revision: https://phabricator.kde.org/D15505