Major step for build process

- updated to 0.4.0
- corrected license for npm
- update to gulp 4
- updated all dependencies
- new and removed packages, see `package.json`
- changed file structure, renamed `pages` to `screen` and new `data` 
folder
- complete change of `gulpfile.js` with similar output

source/screens/demo/error/400.njk

@@ -0,0 +1,23 @@
+<!-- page-error-404.njk -->
+{% set pageId = "400" %}
+{% set pageClass = "body_status" %}
+
+{% extends "demo/maintenance.njk" %}
+
+{% block title %}{{ pageId }}{% endblock %}
+{% block head %}
+{{ super() }}
+{% endblock %}
+
+{% block body_content %}
+<main class="main_site">
+  <h1>{{ pageId }}</h1>
+  <h3>Client-Fehler</h3>
+  <p>Fehlerhafte Anfrage! <dfn>Bad Request</dfn></p>
+  <blockquote cite="https://de.wikipedia.org/wiki/HTTP-Statuscode#4xx_.E2.80.93_Client-Fehler">
+    <p>Die Anfrage-Nachricht war fehlerhaft aufgebaut.</p>
+    <p class="quote_source"><a href="https://de.wikipedia.org/wiki/HTTP-Statuscode#4xx_.E2.80.93_Client-Fehler">Wikipedia</a></p>
+  </blockquote>
+</main>
+{{ super() }}
+{% endblock %}