asset image: use cache-buster (_dc) parameter on image for preview tab -> for legacy startup

git-svn-id: http://www.pimcore.org/svn/pimcore/private/core/trunk@3092 1f8fe7d8-47f0-464c-8d0a-336f4953ab05
build auto version ...

git-svn-id: http://www.pimcore.org/svn/pimcore/private/core/trunk@3093 1f8fe7d8-47f0-464c-8d0a-336f4953ab05