Add virtual Scene::scenePainter method

Summary:
So far EffectsHandlerImpl directly accessed SceneQPainter::painter
through a dynamic cast. If in future the QPainter based compositor should
be moved into a plugin we cannot access it through a dynamic cast.

To solve this problem the painter method is moved into Scene as
a virtual method returning a sane default value.

Reviewers: #kwin, #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D7214