Fix clang warning about vtables being emitted in every translation unit.
Summary: Pure interfaces don't get treated well by C++ compilers, better have a .cpp file for the dtor. This required moving the header to one of the actual libs (viewer, since it's only used there). Test Plan: Compiles Reviewers: #kde_pim Differential Revision: https://phabricator.kde.org/D1376
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