Improve descriptions

[adi-de-masi]

Is your feature request related to a problem? Please describe.
Identifying the exact purpose of a description in the <descriptions> is not easy and leaves room for interpretation. There is only an optional "title" element to explain the content in the description element. As real estate platform, it is crucial to know without any doubt what the free texts are meant to be.

Describe the solution you'd like
Replace the <descriptions> list of texts with a more concise type. Here is a suggestion:

<texts>
    <title>Einfacher Vermarktungstitel</title>
    <excerpt>Kurzfassung des Angebots</excerpt>
    <situation>Die lage des Angebots</situation>
    <description><![CDATA[<p>Lorem ipsum dolor sit amet, consectetur adipiscing elit. Neque solum ea communia, verum etiam paria esse dixerunt.</p>]]></description>
</texts>

Additional context
Note that the custom formatting in the <description> above is not part of this feature request, see #45 for that.

[jaggli]

The other day I was thinking if we should group all language specific tags under one parent, something like the following (I did not think it through, I'll just drop it here, for reference)

<property id="property123">
  <!-- ... -->
  <localizations>
    <localization lang="de">
      <name>Einfacher Vermarktungstitel</name>
      <excerpt>Kurzfassung des Angebots</excerpt>
      <description><![CDATA[<p>Lorem ipsum dolor sit amet</p>]]></description>
      <location><![CDATA[<p>Lorem ipsum dolor sit amet</p>]]></location>
      <equipment><![CDATA[<p>Lorem ipsum dolor sit amet</p>]]></equipment>
      <urls>
        <url>http://mypropertyhomepagehere.ch</url>
        <url>http://realestatewebsite.ch</url>
      </urls>
    </localization>
    <localization lang="en">
      <name>Simple name</name>
      <excerpt>Short excpert</excerpt>
      <description><![CDATA[<p>Lorem ipsum dolor sit amet</p>]]></description>
    </localization>
  </localizations>
  <publishers>
    <!-- 
      generic freestyle key/values
      schema: sequence
    -->
    <publisher id="homegate">
      <options>
        <option key="website-title" lang="it">Mamma mia</option>
        <option key="website-title" lang="en">Mamma mia</option>
        <option key="form-type">full</option>
        <option
          key="promotion"
          lang="en"
          start="2018-10-10T17:00:00Z"
          expiration="2018-10-10T17:00:00Z"
        >
          top
        </option>
        <option
          key="promotion"
          lang="de"
          start="2018-10-10T17:00:00Z"
          expiration="2018-10-11T17:00:00Z"
        >
          top
        </option>
      </options>
    </publisher>
  </publishers>
  <!-- ... -->
 </property>

[jaggli]

done in feature/refactoring branch