Work Started on OMDoc 1.3

We have started work on the [OMDoc1.3] specification. This is to be a maintenance release with addresses immediate needs from the [wiki:OMDocToolsProjects OMDoc applications] that have been the focus of development in the last years. The main work items are * a new presentation system (see #1616) * a new syntax for structure sharing and referencing (see #1624) * a new metadata framework (see #1625) * corresponding metadata ontolgoies (see #1626)