Reorganize projects so that they can be built with Maven/Tycho.

Note that plug-in org.lh.dmlj.schema.editor is renamed to
org.lh.dmlj.schema.editor.core to avoid a conflict with the feature
artifact (id: org.lh.dmlj.schema.editor); we need to keep the id of the
feature unmodified so that users can use Eclipse's update feature to
switch to the new version.  As a consequence, the fully qualified name
of the 'import' extension point has changed from
org.lh.dmlj.schema.editor.import to
org.lh.dmlj.schema.editor.core.import.