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/AboutDialog.cpp

@@ -300,7 +300,7 @@ AboutDialog::AboutDialog(wxWindow * parent)
 
    S.Id(wxID_OK)
       .Prop(0)
-      .AddButton(XO("OK"), wxALIGN_CENTER, true);
+      .AddButton(XXO("OK"), wxALIGN_CENTER, true);
 
    Fit();
    this->Centre();