Add move constructor and appropriately overloaded assignment operator to
rtl::Reference, and add basic unit tests for the reference counting of
rtl::Reference.

Change-Id: Ia7ff5d786bdf3b17709cec06608c91e22379746c
Reviewed-on: https://gerrit.libreoffice.org/19762Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>