v1.6.0 : var inside header or footer

[philippe-levan]

Add the ability to put variables inside the header or the footer of a document.

Changes

• allow put variables inside headers and footers
• fix a bug when a variable is both inside the text content and inside a table (it should not arrive, but it is fixed)
• a new unit test system based on PDF converted to text in order to test contents that are not converted to text with a simple saveAs

Checklist

• If code changed were made, they have been tested.
• This pull request fixes an issue.
• This pull request adds a feature.
• The code has been tested.