Send Unicode data as UTF-8 text when using DDE-based IPC

This is a more hackish but more compatible solution to the problem of
data sent using wxIPC_UTF8TEXT format being simply lost when using DDE
for IPC classes. We must use CF_TEXT for the DDE to pass our data, but
we can try to decode it as UTF-8 in the client and assume it was sent in
this format if it worked. This obviously suffers from false positives as
any ASCII string will still be assumed to be UTF-8, but there shouldn't
be any real harm coming from this.

This change also makes sending data in wxIPC_UTF{16,32}TEXT formats work
as well by converting it to UTF-8.

Update the sample to call Advise() with both wxIPC_UTF{8,16}TEXT formats
and remove the now unnecessary wxDDEConnection::m_dataType member.

Closes #17900.

docs/changes.txt

@@ -124,6 +124,10 @@ All (GUI):
 - Allow changing tooltip text for button allowing to enter a new string
   in wxPGArrayEditorDialog.
 
+wxMSW:
+
+- Fix passing Unicode strings via wxIPC when using DDE.
+
 
 3.1.2: (released 2018-12-10)
 ----------------------------