MainWindow: Use params pattern for _init

This makes MainWindow consistent with most other classes in Maps.

https://bugzilla.gnome.org/show_bug.cgi?id=740647

src/application.js

@@ -212,7 +212,8 @@ const Application = new Lang.Class({
                                                                              'icons']));
         let overlay = new Gtk.Overlay({ visible: true, can_focus: false });
         notificationManager = new NotificationManager.NotificationManager(overlay);
-        this._mainWindow = new MainWindow.MainWindow(this, overlay);
+        this._mainWindow = new MainWindow.MainWindow({ application: this,
+                                                       overlay: overlay });
         this._mainWindow.connect('destroy', this._onWindowDestroy.bind(this));
     },
 

src/mainWindow.js

@@ -58,16 +58,16 @@ const MainWindow = new Lang.Class({
                         'layersButton',
                         'favoritesButton' ],
 
-    _init: function(app, overlay) {
-        this.parent();
+    _init: function(params) {
+        this._overlay = params.overlay;
+        delete params.overlay;
 
-        this._configureId = 0;
+        this.parent(params);
 
-        this.application = app;
-        this._overlay = overlay;
+        this._configureId = 0;
 
         this.mapView = new MapView.MapView();
-        overlay.add(this.mapView);
+        this._overlay.add(this.mapView);
 
         this.mapView.gotoUserLocation(false);