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