...by explicitly defaulting the copy/move functions (and, where needed in turn,
also a default ctor) for classes that have a user-declared dtor that does
nothing other than an implicitly-defined one would do, but needs to be user-
declared because it is virtual and potentially serves as a key function to
emit the vtable, or is non-public, etc.  There appears to be no good reason for
these classes to have a deleted copy assignment op, so just change that to
implicitly-defined, too.

Change-Id: I65fb6ea78a13b7b3d237529613c0489c8cf50614
Reviewed-on: https://gerrit.libreoffice.org/58119
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>