D7279: Do not leak symbols of pimpl classes, protect with Q_DECL_HIDDEN

[David Faure]

dfaure added a reviewer: mlaurent.

REPOSITORY
  R236 KWidgetsAddons

REVISION DETAIL
  https://phabricator.kde.org/D7279

To: kossebau, cfeck, dfaure, vkrause, mlaurent
Cc: #frameworks