image thumbnail: data collection in separate method

git-svn-id: http://www.pimcore.org/svn/pimcore/private/core/trunk@2762 1f8fe7d8-47f0-464c-8d0a-336f4953ab05
image thumbnails: high resolution -> supporting Apple's prescribed high-resolution modifier (@2x) in filename

git-svn-id: http://www.pimcore.org/svn/pimcore/private/core/trunk@2763 1f8fe7d8-47f0-464c-8d0a-336f4953ab05