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.

src/export/ExportMP2.cpp

@@ -162,7 +162,7 @@ void ExportMP2Options::PopulateOrExchange(ShuttleGui & S)
          S.StartMultiColumn(2, wxCENTER);
          {
             S.TieNumberAsChoice(
-               XO("Bit Rate:"),
+               XXO("Bit Rate:"),
                {wxT("/FileFormats/MP2Bitrate"),
                 160},
                BitRateNames,