Fix clang warning about the ObjectTreeSourceIf vtable 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: knauss, KDE PIM
Reviewed By: knauss, KDE PIM
Subscribers: knauss
Differential Revision: https://phabricator.kde.org/D1376