Tab stop is added as child of style:style element

[connium]

Describe the bug
Tab stops which are defined on a paragraph style, are not rendered in a OpenDocument consumer.
Reason for this behavior is that the style:tab-stop element is added as a child of the style:style element. The style:tab-stops element inside the style:paragraph-properties element is empty.

<style:paragraph-properties>
  <style:tab-stops/>
</style:paragraph-properties>
<style:tab-stop style:position="2mm"/>

To Reproduce
Steps to reproduce the behavior:

1. Create a TextDocument
2. Add a ParagraphStyle to the common styles of the document
3. Add a tab stop to the ParagraphStyle
4. Export the document

Expected behavior
This is what the serialized style should look like:

<style:paragraph-properties>
  <style:tab-stops>
    <style:tab-stop style:position="2mm"/>
  </style:tab-stops>
</style:paragraph-properties>

Environment (please complete the following information):

• OS: Ubuntu
• Node.js version: 8.16.0
• Version: 0.9.0