Add new layout tests.
Layout tests can be added in layouttests.xml. A file is constructed from
ODF fragments and the loaded and layed out document is checked.
Only a few simple dimensions for are currently checked, more tests will follow.
Use more commen xml prefixes and add typing information to the js.
Use odf.OdfUtils.parseLength and odf.Namespaces.lookupNamespaceURI instead of duplicated code.