Migrate old chains...

... Copy old Chains files to Macros, once only per session, never overwriting.

Leave old files in place in case the user downgrades their Audacity.

When removing a Macro, remove also any like-named legacy chain.

src/BatchCommands.h

@@ -79,6 +79,7 @@ class MacroCommands final {
    bool IsMono();
 
    // These commands do not depend on the command list.
+   static void MigrateLegacyChains();
    static wxArrayString GetNames();
    static wxArrayString GetNamesOfDefaultMacros();