[kstyle] Implement application unpolish to delete ShadowHelper

Summary:
Another change mostly for KWin (Wayland compositor). KWin destroys it's
internal Wayland connection prior to the QStyle getting destroyed. As
the ShadowHelper initialized Wayland objects those would be destroyed
after the connection is destroyed. With latest Wayland library this
would cause a crash.

Thus unpolish is implemented in the style and deletes the ShadowHelper.
KWin can on tear down invoke the unpolish and thus make sure it doesn't
crash.

CCBUG: 372001

Reviewers: #plasma, broulik, hpereiradacosta

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D3240