Unpolish QStyle on QApp prior to destroying internal Wayland connection

Summary:
The QStyle might interact with our internal Wayland connection. If the
cleanup happens after destroying our internal Wayland connection KWin
will crash at tear-down.

With this change the QStyle can perform cleanup already prior to the
deconstruction of the Wayland connection.

CCBUG: 372001

Reviewers: #kwin, #plasma_on_wayland

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D3241