Figure out nested wayland dismantle

Summary:
Fixes a crash we would get, we're changing 2 main things:

• release the event queue first, otherwise we'd serve events to

disappearing objects.

• destroy the connection thread object after the thread has quit.

Test Plan: KWin doesn't explode when closing.

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: davidedmundson, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D28340