Export `sequence`: Allow to export non-ordered nodes

[MartinKl]

When exporting sequences, the exporter can currently only find annotations on the ordered nodes directly. It should be extended to finding annotations on edges that are connected to the ordered nodes, e. g. via Coverage edges. This feature is not required is not essential, since annotations can currently also be mapped to the ordered nodes via other modules, but it would significantly downsize workflows and provide a lower threshold for less experienced users.