Refactoring HtmlConverter: htmlToText Methods

This patch refactors the htmlToText methods of the HtmlConverter helper class:

* htmlize text line by line to clarify the procedue and reduces overall memory usage
* different approach to linkification which is more transparent and less error-prone
* remove functional duplicates

Beforehand some code was moved around to improve code readability by

* grouping methods of the same domain (FileUtils / StringUtils)
* moving used-only-once static vars to the place where they are used (mostly regex patterns)

The code was tested with the given test cases.