Translate "Message" as default title of message box...

... This required a sweeping change of all calls to wxMessageBox!  But it seems
safe to me, despite the great number of touched files.

src/commands/CommandManager.cpp

@@ -83,7 +83,6 @@ CommandManager.  It holds the callback for one command.
 #include <wx/hash.h>
 #include <wx/intl.h>
 #include <wx/log.h>
-#include <wx/msgdlg.h>
 #include <wx/tokenzr.h>
 
 #include "../AudacityException.h"
@@ -1710,7 +1709,7 @@ bool CommandManager::HandleXMLTag(const wxChar *tag, const wxChar **attrs)
 void CommandManager::HandleXMLEndTag(const wxChar *tag)
 {
    if (!wxStrcmp(tag, wxT("audacitykeyboard"))) {
-      wxMessageBox(wxString::Format(_("Loaded %d keyboard shortcuts\n"),
+      AudacityMessageBox(wxString::Format(_("Loaded %d keyboard shortcuts\n"),
                                     mXMLKeysRead),
                    _("Loading Keyboard Shortcuts"),
                    wxOK | wxCENTRE);

src/commands/CommandTargets.h

@@ -24,11 +24,11 @@ should be reference-counted.
 
 #include "../MemoryX.h"
 #include <wx/string.h>
-#include <wx/msgdlg.h>
 #include <wx/statusbr.h>
 #include "../widgets/ProgressDialog.h"
 #include "../commands/ResponseQueue.h"
 #include "../src/Project.h"
+#include "../widgets/ErrorDialog.h"
 
 /// Interface for objects that can receive command progress information
 class CommandProgressTarget /* not final */
@@ -96,14 +96,14 @@ public:
    void Update(const wxString &) override {}
 };
 
-/// Displays messages from a command in a wxMessageBox
+/// Displays messages from a command in an AudacityMessageBox
 class MessageBoxTarget final : public CommandMessageTarget
 {
 public:
    virtual ~MessageBoxTarget() {}
    void Update(const wxString &message) override
    {
-      wxMessageBox(message);
+      AudacityMessageBox(message);
    }
 };