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/pages/demo/error/500.njk

@@ -1,23 +0,0 @@
-<!-- page-error-500.njk -->
-{% set pageId = "500" %}
-{% 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>Server-Fehler</h3>
-  <p>Allgemeiner Server Fehler!!! <dfn>Internal Server Error</dfn></p>
-  <blockquote cite="https://de.wikipedia.org/wiki/HTTP-Statuscode#5xx_%E2%80%93_Server-Fehler">
-    <p>Dies ist ein „Sammel-Statuscode“ für unerwartete Serverfehler.</p>
-    <p class="quote-author"><a class="line-link" href="https://de.wikipedia.org/wiki/HTTP-Statuscode#5xx_%E2%80%93_Server-Fehler">Wikipedia</a></p>
-  </blockquote>
-</main>
-{{ super() }}
-{% endblock %}