Configure the icon of the main window

Fixes the incorrect application icon in linux/gnome-shell. See
BoostIO/Boostnote#251

lib/main-window.js

@@ -17,7 +17,8 @@ const mainWindow = new BrowserWindow({
   webPreferences: {
     zoomFactor: 1.0,
     blinkFeatures: 'OverlayScrollbars'
-  }
+  },
+  icon: path.resolve(__dirname, '../resources/app.png')
 })
 
 const url = path.resolve(__dirname, './main.html')