cli_ure: adapt destructors in cli_uno library to "C++/CLI"

commit 4b56d82c7d20ba5897d87aaf7fc94da5356b8eec converted the cli_uno
library from "Managed C++" to "C++/CLI", but forgot one detail:

The destructors on "ref" classes were mapped to Finalize() methods in
the old syntax, but the new one maps them to Dispose() methods, which
are only invoked on stack-allocated objects.  Presumably this omission
results in leaking of native C++ UNO objects.

Reading the C++/CLI documentation i get the impression that:
1) the destructor should explicitly call the finalizer
2) the CLR will not call the finalizer itself iff the destructor is
   invoked

http://msdn.microsoft.com/en-us/library/ms235315.aspx
http://msdn.microsoft.com/en-us/library/ke3a209d%28v=vs.110%29.aspx

Change-Id: I509d9b69a399c3d7d6597060ab9b7c78c5916e11
Reviewed-on: https://gerrit.libreoffice.org/11132
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>