Refs #15214. Copy T3 vars BE_USER, BE_USER_UID, T3DATA_UID to Fake STORE_TYPO3 (via form/CLIENT_TYPO3VARS)

Fixes #14292. Upload: mode=required broken for advanced upload (non primary column) after first save

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

Implemented: file upload check for mime type and max file size.
File.php, AbstractBuildForm.php: Implement FE_FILE_MIME_TYPE_ACCEPT and FE_FILE_MAX_FILE_SIZE

* Recode the '_link' notation for download
* Add 'ZIP' as export format
* Add 'parameter' to wkhtmltopdf
* Implemented 'SYSTEM_STORE / SYSTEM_DOWNLOAD_POPUP' for detecting if there are download links on the page. This eleminates passing parameters through dozens of functions.
* Report/Columntype '_dDownload' broken!
Coding.md: described `download` from a coding point of view.
Manual.rst: Update '_link' to latest notation of 'download'.
download.php: implemented catching of 'Undefined index'. Added further exceptions.
Download.php: Added cache=off for downloading. Rename getFile() to getElement(). Make getElement() more generic. Add zipFiles(). Implement 'downloadMode' in doElements().
Html2Pdf.php: recode to new download notation. Parameter to wkhtmltopdf implemented.
Link.php: New TOKEN_* and NAME_DOWNLOAD_*, NAME_FILE. Move TOKEN_* to Constants.php. Will be used in Download.php too. Implemented 'SYSTEM_STORE / SYSTEM_DOWNLOAD_POPUP'. Implemented BASE64 encoding of multiple 'U' and 'u'.
Report.php: Implemented 'SYSTEM_STORE / SYSTEM_DOWNLOAD_POPUP'.
Sip.php: implement debugSip() to show Sip. New: base64 encoded parameter will be shown in clear.
Store.php: some functions missed keyword 'static'. getVar() and getStore automatically decode base64 parameter.
QuickFormQuery.php: Implemented 'SYSTEM_STORE / SYSTEM_DOWNLOAD_POPUP'.

#3180=#2081 dynamicUpdate, rearrange 'input hidden' elements from checkboxes, radios to top of form.

CODING.md: add some doc about STORE_ADDITIONAL_FORM_ELEMENTS.
load.php (#2081): rearrange delivered JSON to fullfill client API.
AbstractBuildForm.php: rename getJsonElementUpdate() to getFormElemenForJson(). Extend getFormElementForJson() to support changes for label and note. Fill store STORE_ADDITIONAL_FORM_ELEMENTS with 'hidden input' elments.
QuickFormQuery.php: implement Client JOSN API update.

PROTOCOL.md: added 'the attribute' in one sentence

#3253: STORE_TYPO3: steht waehrend Update / Save nicht zur Verfuegung - wird aber benoetigt. Bsp: feUser, beUser, pageId, ...

CODING.md: described implementation.
Store.php: fixed setting of STORE_SYSTEM / showDebugInfo during API call.

Coding.md: Beschreibung in Deutsch wie die Formularelemente , rekursiv, gerendert werden.
AbstractBuildForm.php: new function buildTemplateGroup()
BuildFormBootstrap.php, BuildFormPlain.php, BuildFormTable.php: Preparation for new optional TemplateGroup wrap.
QuickFormQuery.php: update comments.
formEditor.sql: column 'type' definition extended - new 'templateGroup' enum. FormElement 'type' updated to new container element 'templateGroup'

Support.php, AbstractBuildForm.php: new function falseemptyToZero().
Constants.php: New UPLOAD_MODE_*
Evaluate.php: fixed bug in 'skip' logic of evaluate->parseArray()
Save.php: doUpload() now returns $modeUpload, which are needed in doUploadSlave(), rewrote logic of doUploadSlave(),

CODING.md: update docs for situation 'new record already saved'.
FillStoreForm.php: In case of form load with r=0, the submitted form element names are does not contain the effective record id - workaround implemented.
Session.php: refactored clear(), create unsetItem().
Sip.php: new buildParamStringFromArray(), updateSipToSession().
Store.php: For r=0 SIPs always add a uniqe parameter.
AbstractBuildForm.php: modified message for subrecords on new record.
QuickFormQuery.php: create unique SIP on form load.

CODING.md: edited to be more clearly.
AbstractBuildForm.php: doc
Save.php: If there is no upload, do not save the filename again (which needs to be calculated instead of using the sip)

Store.php: fills arrays direct in $_SESSION - not sure if this works.
AbstractBuildForm.php: buildFile() extended to create hidden sipUplaod element.
BuildFormBootstrap.php: support different actions on calling file.php
File.php, Save.php: rewrote whole logic of uploading files. See CODING.md

QfqForm.js: Buttons should behave as described in CODING.md under UI.

Support.php: rewrote appendTypo3ParameterToArray() to use Typo3 Store instead of _GET Parameter. Moved randomAlphaNum() to Support.
Link.php: rewrote handling of ToolTip. Renamed doHtmlUrl() to doAnchor().
Utils.php: moved randomAlphaNum() to Support.
Store.php: rewrote fillStoreClient() to be more compatible with phpUnit-test environment. Removed randomAlphaNum() - duplicate in Support.
AnbstractBuildForm.php: rewrote deriveNewRecordUrlFromExistingSip() to use generic Support::appendTypo3ParameterToArray()
Coding.md: Notes for debuggging.