Add AUDACITY_FILE_SUFFIX_EVENT to ExportPCM

This signals a change in the file suffix, and it is received by the Exporter.
However, there is currently no way to change the displayed filename in
the file selection dialog.

This was a step towards a fix for Bug 1355 - "Other uncompressed files" does not (visually) update target file extension according to the chosen "Header" type

src/export/Export.cpp

@@ -266,8 +266,12 @@ void ExportPlugin::InitProgress(std::unique_ptr<ProgressDialog> &pDialog,
 // Export
 //----------------------------------------------------------------------------
 
+
+wxDEFINE_EVENT(AUDACITY_FILE_SUFFIX_EVENT, wxCommandEvent);
+
 BEGIN_EVENT_TABLE(Exporter, wxEvtHandler)
    EVT_FILECTRL_FILTERCHANGED(wxID_ANY, Exporter::OnFilterChanged)
+   EVT_COMMAND( wxID_ANY, AUDACITY_FILE_SUFFIX_EVENT, Exporter::OnExtensionChanged)
 END_EVENT_TABLE()
 
 Exporter::Exporter()
@@ -305,6 +309,23 @@ Exporter::~Exporter()
 {
 }
 
+// Beginnings of a fix for bug 1355.
+// 'Other Uncompressed Files' Header option updates do not update
+// the extension shown in the file dialog.
+// Unfortunately, although we get the new extension here, we
+// can't do anything with it as the FileDialog does not provide
+// methods for setting its standard controls.
+// We would need OS specific code that 'knows' about the system 
+// dialogs.
+void Exporter::OnExtensionChanged(wxCommandEvent &Evt) {
+   wxString ext = Evt.GetString();
+   ext = ext.BeforeFirst(' ').Lower();
+   wxLogDebug("Extension changed to '.%s'", ext);
+//   wxString Name = mDialog->GetFilename();
+//   Name = Name.BeforeLast('.')+ext;
+//   mDialog->SetFilename(Name);
+}
+
 void Exporter::SetFileDialogTitle( const wxString & DialogTitle )
 {
    // The default title is "Export File"

src/export/Export.h

@@ -152,6 +152,10 @@ WX_DEFINE_USER_EXPORTED_ARRAY_PTR(wxWindow *, WindowPtrArray, class AUDACITY_DLL
 //----------------------------------------------------------------------------
 // Exporter
 //----------------------------------------------------------------------------
+
+// For a file suffix change from the options.
+wxDECLARE_EVENT(AUDACITY_FILE_SUFFIX_EVENT, wxCommandEvent);
+
 class  AUDACITY_DLL_API Exporter final : public wxEvtHandler
 {
 public:
@@ -188,6 +192,7 @@ class  AUDACITY_DLL_API Exporter final : public wxEvtHandler
    int GetAutoExportSubFormat();
    int GetAutoExportFilterIndex();
    wxFileName GetAutoExportFileName();
+   void OnExtensionChanged(wxCommandEvent &Evt);
 
 private:
    bool ExamineTracks();

src/export/ExportPCM.cpp

@@ -280,6 +280,14 @@ void ExportPCMOptions::OnHeaderChoice(wxCommandEvent & WXUNUSED(evt))
    ValidatePair(GetFormat());
 
    TransferDataFromWindow();
+
+   // Send the event indicating a file suffix change.
+   // We pass the entire header string, which starts with the suffix.
+   wxCommandEvent event(AUDACITY_FILE_SUFFIX_EVENT, GetId());
+   event.SetEventObject(this);
+   event.SetString(mHeaderChoice->GetString(mHeaderChoice->GetSelection()));
+   ProcessWindowEvent(event);
+
 }
 
 int ExportPCMOptions::GetFormat()