asset: ZIP download: use relative path names to the parent not the full path

git-svn-id: http://www.pimcore.org/svn/pimcore/private/core/trunk@2708 1f8fe7d8-47f0-464c-8d0a-336f4953ab05
asset: ZIP import: do not import all files at ones, only 10 items per request (+ progressbar) -> see zip export

git-svn-id: http://www.pimcore.org/svn/pimcore/private/core/trunk@2709 1f8fe7d8-47f0-464c-8d0a-336f4953ab05