Release v1.2

Changelog:

• Manual: Updated text and dutch translations
• Manual: Updated screenshots for all languages
• Fix #211: Be more robust if no .git file is there
• Fix cmake warnings
• Substancial code cleanups
• Fix reproducable builds
• Fix saving of XRechnung files
• Added a template for XRechnung version 3.0.1
• Fix #218: Add a image mask for reportlab
• Fix tests
• Add checkbox to mark document types to get XRechnung support
• Add a feedback link to the system overview page