document video tag: improved previews (still + animated GIF) + added VP9 support

git-svn-id: http://www.pimcore.org/svn/pimcore/private/core/trunk@4882 1f8fe7d8-47f0-464c-8d0a-336f4953ab05
test-commit

git-svn-id: http://www.pimcore.org/svn/pimcore/private/core/trunk@4883 1f8fe7d8-47f0-464c-8d0a-336f4953ab05