Move MyCopyFile into class FileNames

src/FileNames.cpp

@@ -43,6 +43,32 @@ used throughout Audacity into this one place.
 
 static wxString gDataDir;
 
+bool FileNames::CopyFile(
+   const wxString& file1, const wxString& file2, bool overwrite)
+{
+#ifdef __WXMSW__
+
+   // workaround not needed
+   return wxCopyFile(file1, file2, overwrite);
+
+#else
+   // PRL:  Compensate for buggy wxCopyFile that returns false success,
+   // which was a cause of case 4 in comment 10 of
+   // http://bugzilla.audacityteam.org/show_bug.cgi?id=1759
+   // Destination file was created, but was empty
+   // Bug was introduced after wxWidgets 2.8.12 at commit
+   // 0597e7f977c87d107e24bf3e95ebfa3d60efc249 of wxWidgets repo
+
+   bool existed = wxFileExists(file2);
+   bool result = wxCopyFile(file1, file2, overwrite) &&
+      wxFile{ file1 }.Length() == wxFile{ file2 }.Length();
+   if (!result && !existed)
+      wxRemoveFile(file2);
+   return result;
+
+#endif
+}
+
 wxString FileNames::MkDir(const wxString &Str)
 {
    // Behaviour of wxFileName::DirExists() and wxFileName::MkDir() has