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