Handle the loading state whilst appending many tracks better

1) Update the queue length periodically so that the user can see we're
   working
2) Clear any previous track and display "loading..." instead

lua/playing.lua

@@ -226,7 +226,18 @@ return screen:new {
         end
       end),
       playback.track:bind(function(track)
-        if not track then return end
+        if not track then
+          if queue.loading:get() then
+            title:set { text = "Loading..." }
+          else
+            title:set { text = "" }
+          end
+          artist:set { text = "" }
+          cur_time:set { text = format_time(0) }
+          end_time:set { text = format_time(0) }
+          scrubber:set { value = 0 }
+          return
+        end
         if track.duration then
           end_time:set { text = format_time(track.duration) }
         else