Unmap PlasmaWindowInterface instead of deleting directly

Summary:
The window will be cleaned up automatically once all bound resources
are destroyed. A manual delete though might cause protocol errors on
client side as the unmap and resource destroy happen together and we
are in a race condition.

To ensure that we don't leak the interface is set as the parent, so
on tear-down remaining resources will be destroyed eventually, even
if it triggers a protocol error on the client.

Reviewers: Plasma

Subscribers: plasma-devel

Projects: Plasma

Differential Revision: https://phabricator.kde.org/D1595