make destructors virtual in base classes and only there.
try to implement your class in a way that the automatically generated
copy constructor and copy assignment work out-of-the box.
(we agreed on that a few weeks ago)
git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@8167 a592a061-630c-0410-9148-cb99ea01b6c8
