Pref: default torrent file handler

Before, the app made itself the default torrent file handler automatically, pissing off some of our users. Now, it's not by default, and you can change it in the prefs.

src/renderer/controllers/prefs-controller.js

@@ -1,5 +1,6 @@
 const {dispatch} = require('../lib/dispatcher')
 const State = require('../lib/state')
+const ipcRenderer = require('electron').ipcRenderer
 
 // Controls the Preferences screen
 module.exports = class PrefsController {
@@ -41,6 +42,9 @@ module.exports = class PrefsController {
   // All unsaved prefs take effect atomically, and are saved to config.json
   save () {
     var state = this.state
+    if (state.unsaved.prefs.isFileHandler !== state.saved.prefs.isFileHandler) {
+      ipcRenderer.send('setDefaultFileHandler', state.unsaved.prefs.isFileHandler)
+    }
     state.saved.prefs = Object.assign(state.saved.prefs || {}, state.unsaved.prefs)
     State.save(state)
   }