Cleanup in API of DiagramBehavior

*Modifier of externally called methods changed to protected if possible
*Kept diagram editor input for referencing diagram URI because
substitution would mean replacement with 2 fields (URI and
diagramTypeProviderId)
*Took over JavaDoc from old DiagramEditor

Change-Id: I7f1af3929dd66b6573cd02449ffa41a2b1e101c6