Do not leak symbols of pimpl classes, protect with Q_DECL_HIDDEN
Summary:
Nested classes inherit the visibility of their outer class,
so all the KClass::Private symbols have also been exported
if the ones of KClass were.
See also https://phabricator.kde.org/D7279
Test Plan:
Ran nm -DC to list symbols
Only changed visibility of classes defined in .cpp files, so no client code should be affected.
Reviewers: Plasma, dfaure
Reviewed By: dfaure
Subscribers: plasma-devel, Frameworks
Tags: Plasma on Wayland, Frameworks
Differential Revision: https://phabricator.kde.org/D7304