Limit the filesystem to download folder only

* Change the "--filesystem" flag from "home" to "xdg-download".
  That means Signal can only access the XDG Download folder of the
  user instead of full access to home folder.

* Resolve flathub#153.

org.signal.Signal.json

@@ -17,7 +17,7 @@
         "--device=all",
         "--nodevice=kvm",
         "--nodevice=shm",
-        "--filesystem=home",
+        "--filesystem=xdg-download",
         "--talk-name=org.freedesktop.Notifications",
         "--talk-name=org.kde.StatusNotifierWatcher",
         "--own-name=org.kde.StatusNotifierItem-2-1"
@@ -70,4 +70,4 @@
             ]
         }
     ]
-}
+}