#3218 / download.php / export

* 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'.