Change XO to XXO in many more places, with no effects at all...

... because the two macros have the same expansion, and are both checked for
in the --keyword arguments passed to msgfmt by locale/update_po_files.sh.

This commit makes ONLY such changes, and comments in Internat.h.  It is big
but quite harmless.

The intention is to introduce a type distinction in a later release, by defining
XXO differently.  XXO is used where & characters in strings (for hotkeys of menu
items or control prompts) are permitted, XO where not.

lib-src/mod-null/ModNullCallback.cpp

@@ -146,12 +146,12 @@ void RegisterMenuItems()
       ( FinderScope( ident ), Section( wxT("NullModule"),
          Command(
             _T("A New Command"), // internal name
-            XO("1st Experimental Command..."), //displayed name
+            XXO("1st Experimental Command..."), //displayed name
             ModNullFN( OnFuncFirst ),
             AudioIONotBusyFlag() ),
          Command(
             _T("Another New Command"),
-            XO("2nd Experimental Command"),
+            XXO("2nd Experimental Command"),
             ModNullFN( OnFuncSecond ),
             AudioIONotBusyFlag() )
       ) )