Improve text converters to take into account paragraphs and headings

For now the OOo2TextConverter doesn't take into account paragraphs and headings.
Which means that the following content in an .odt document:

This is a document with
a new line and a paragraph.

Paragraph 1 heading
Content of the paragraph 1. Content of the paragraph 1. Content of the paragraph 1. Content of the paragraph 1.

will be converted to the string:

This is a document with a new line and a paragraph. Paragraph 1 heading Content of the paragraph 1. Content of the paragraph 1. Content of the paragraph 1. Content of the paragraph 1.

The MSOffice2TextConverter does detect paragraphs but not headings, it could be nice to have an empty line before each hading to make the difference between a paragraph (= 1 new line) and a heading (= 2 new lines) when reading the converted text.