[IMP/FIX] playground: fix and improve various things

The playground hasn't been touched for a while and a few things were
broken before this commit:
- Exporting as a standalone web application was broken because it still
attempted to load the templates like it was done in owl 1
- Resizing the editor tabs still tried to use `this.trigger`

While fixing the export feature, the files needed by the app were
factored out of hardcoded strings and into real files, as this makes it
easier to maintain, since these files get syntax highlighting.

The samples received the same treatment: there is now a `samples` folder
containing all the samples, it also contains a jsconfig, giving
autocompletion on owl functions while editing the files, and allowing
the owl import to look how it would when using owl as a node_module.
In the browser, this import is translated to the relative path of the
owl module using an import map.

tools/playground/standalone_app/app.py

@@ -0,0 +1,28 @@
+#!/usr/bin/env python3
+
+import threading
+import time
+
+from http.server import SimpleHTTPRequestHandler, HTTPServer
+
+httpd = None
+def start_server():
+    global httpd
+    SimpleHTTPRequestHandler.extensions_map['.js'] = 'application/javascript'
+    httpd = HTTPServer(('0.0.0.0', 3600), SimpleHTTPRequestHandler)
+    httpd.serve_forever()
+
+url = 'http://127.0.0.1:3600'
+
+if __name__ == "__main__":
+    print("Owl Application")
+    print("---------------")
+    print("Server running on: {}".format(url))
+    threading.Thread(target=start_server, daemon=True).start()
+
+    while True:
+        try:
+            time.sleep(1)
+        except KeyboardInterrupt:
+            httpd.server_close()
+            quit(0)

tools/playground/standalone_app/index.html

@@ -0,0 +1,22 @@
+<!DOCTYPE html>
+<html lang="en">
+  <head>
+    <meta charset="utf-8">
+    <title>OWL App</title>
+    <link rel="icon" href="data:,">
+
+    <link rel="stylesheet" href="app.css">
+    <script type="importmap">{ "imports": { "@odoo/owl": "./owl.js" } }</script>
+    <script type="module" src="app.js"></script>
+  </head>
+  <body>
+    <pre>
+This app requires a web server, simply opening the html file using your browser will not work.
+You can run a simple web server with one of the following:
+- if you have python3 installed, run app.py with python3 or run 'python3 -m http.server' in the app folder
+- if you have node/npm installed, run 'npx serve' in the app folder
+- if you have php installed, run 'php -S localhost:8080 -t .' in the app folder
+    </pre>
+    <script>if (window.location.protocol !== "file:") document.querySelector("pre").remove()</script>
+  </body>
+</html>