Enh66: Windows and Linux still Non-Modal

As we EndModal on windows, we must ShowModal too.

src/Menus.cpp

@@ -6602,8 +6602,14 @@ void AudacityProject::OnRemoveTracks()
 
 void AudacityProject::OnAbout()
 {
+#ifdef __WXMAC__
    // Modeless dialog, consistent with other Mac applications
    (safenew AboutDialog{ nullptr })->Show(true);
+#else
+   // Windows and Linux still modal.
+   AboutDialog dlog(this);
+   dlog.ShowModal();
+#endif
 }
 
 void AudacityProject::OnHelpWelcome()