update player window title to torrent name (Windows, Linux)

This also moves all the state “diffing” for purposes of updating the
app’s window via Electron APIs into one function updateElectron().

renderer/state.js

@@ -31,6 +31,7 @@ module.exports = {
     duration: 1, /* seconds */
     mouseStationarySince: 0 /* Unix time in ms */
   },
+  prev: {}, /* used for state diffing in updateElectron() */
 
   /* Saved state is read from and written to a file every time the app runs.
    * It should be simple and minimal and must be JSON.