asset preview optimizations

git-svn-id: http://www.pimcore.org/svn/pimcore/private/core/trunk@3849 1f8fe7d8-47f0-464c-8d0a-336f4953ab05
PIMCORE-2090 - Cannot create a new folder under documents

git-svn-id: http://www.pimcore.org/svn/pimcore/private/core/trunk@3850 1f8fe7d8-47f0-464c-8d0a-336f4953ab05
build auto version (2948) ...

git-svn-id: http://www.pimcore.org/svn/pimcore/private/core/trunk@3851 1f8fe7d8-47f0-464c-8d0a-336f4953ab05