Refs #12636. Form: a) Single Tick encode for {{10.line.content::s}}, b) new encoding type 'single tick'. Part a implemented. Test missing.

Refs #12615. Implements silent HEIC/HEIF conversion to png. Write a note in realease notes, to install 'libheif-examples'.

Fixed a lot of unit tests, which did not passed on webwork16/crose/qfq. In test hardcoded 'http://xxxxxxxxxxxxxxxxxxxxxxxxxxxx/qfq/' makes trouble. CR did not find the place (qfq.json baseUrl tried) to adjust for phpUnit Tests.

Fix problem: /var/www/html/my/typo3conf/ext/qfq/Classes/Core/Form/FormAction.php / Line: 394 / Undefined index: saveFormJson
Frage 1: FE_SAVE_FORM_JSON /FE_SAVE_FORM_JSON_NAME werden an keiner anderen Stelle im Code verwendet - wird der Code noch benoetigt?
Frage 2: Wird wirklich bei jeder action element geprueft ob json erzeugt werden soll - macht das Sinn?

REST.rst: Add example with `x-Api-Key`. Add 'x-Api-Key' as processed header. Strip white spaces from value.

Refs #12145 create form with json form editor. Fix unicode error. Fix form file sync (even though disabled). Throw errors if json encode/decode fails. Put form backups into folder named by form id.

Fixes #11265. Checked for BS and plain, active & disabled, with or without glyph. Code refactored to use more of Link class.

WIP: DD wird noch span gewrapped. Ab jetzt Umstellung auf "renderLink wird nicht rekursiv aufgerufen".

WIP: es wird nur noch ein 'class' attribute gerendert:ok. Next Task: alles mit div wrappen, anstatt <span>