Support wxMenuBar forced size which becomes DPI-aware

[KeyWorksRW]

Description:

Code generation for menus is not DPI-aware. It would be useful if wxMenuBar had an option to force image size which we could automatically convert to DPI-aware size. We actually already do this ourselves in our NavPopupMenu class:

    auto dpi_size = wxGetFrame().FromDIP(wxSize(16, 16));
    menu_item = sub_menu->Append(id, label);
    menu_item->SetBitmap(bndl_function(dpi_size.x, dpi_size.y));

This would also help with wxWidgets Art images where menu images can be either 16x16 or 20x20. It would also mean that wxUiEditor menus that are part of the main toolbar would be much more clear on high-resolution displays now that we are switching all artwork to SVG which scales to any resolution.