Created 2.8.x branch
VersionCompare inside addon.precheck.inc.php use a new function to avoid conflicts within wrong results comparing versionnumbers/-strings.Also remove PHP-Code inside "calender-system.css" (line 36) .
Added reload option to Add-on section and removed Admin tool reload
fixed wrong redirect path and added Id tag
Added option to perform pre-installation checks to test requirements of Add-Ons