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().

main/ipc.js

@@ -32,6 +32,10 @@ function init () {
     windows.main.setFullScreen(!windows.main.isFullScreen())
   })
 
+  ipcMain.on('setTitle', function (e, title) {
+    windows.main.setTitle(title)
+  })
+
   ipcMain.on('log', function (e, message) {
     console.log(message)
   })