Refactor: Use own model for header and footer, improve XML escaping

[d22]

Closes #302

• Extract XML escaping logic from Worksheet.java to own helper class. This allows easier reuse, better testing, and reduces the complexity of Worksheet.java
• Create new Class MarginalInformation which represents the header and footer. This reduces duplicated code and makes testing easier.
• Escape header and footer text, this closes #302

Thank you for reviewing and I hope you consider merging this PR.

Br Jonas

[ochedru]

Thank you for your PR! Merging right now.