Let the download list own its downloads through a shared pointer.

A multicall could then dispatch commands on an erased download.

src/ui/download.h

@@ -2,6 +2,7 @@
 #define RTORRENT_UI_DOWNLOAD_H
 
 #include <list>
+#include <memory>
 #include <torrent/peer/peer.h>
 
 #include "display/manager.h"
@@ -36,7 +37,7 @@ public:
     DISPLAY_MAX_SIZE
   } Display;
 
-  Download(core::Download* d);
+  Download(const std::shared_ptr<core::Download>& d);
   ~Download();
 
   void                activate(display::Frame* frame, bool focus = true);