Destroy all kwayland objects created by registry when it is destroyed
Summary:
Currently one has to connect every object manually to connectionDied,
which is something we can do for them.
If the user also has a connection, the second will just no-op.
This fixes objects that linger longer than the QApp.
Reviewers: graesslin
Reviewed By: graesslin
Subscribers: graesslin, plasma-devel, Frameworks
Tags: Plasma on Wayland, Frameworks
Differential Revision: https://phabricator.kde.org/D6727