Merge pull request #2333 from btel/applayout_widget

Implement AppLayout widget

Author: SylvainCorlay

docs/source/examples/Index.ipynb

@@ -36,7 +36,8 @@
     "- [Widget Basics](Widget%20Basics.ipynb)  \n",
     "- [Widget Events](Widget%20Events.ipynb)  \n",
     "- [Widget List](Widget%20List.ipynb)  \n",
-    "- [Widget Styling](Widget%20Styling.ipynb)    \n",
+    "- [Widget Styling](Widget%20Styling.ipynb)\n",
+    "- [Layout Templates](Layoutt%20Templates.ipynb)\n",
     "- [Widget Custom](Widget%20Custom.ipynb)\n",
     "- [Widget Asynchronous](Widget%20Asynchronous.ipynb): how to pause and listen in the kernel for widget changes in the frontend."
    ]