[Plato] Change format of plan to container file

[kraxner]

At the moment sample objects and results are stored in the XML as Base64 encoded bytestreams. On the one hand this is nice, because everything is there in plain text, on the other hand this plain text is hardly readable and accessible with editors if bigger bytestreams are included.

We might consider to switch do a new container format, similar to Microsoft Office's docx and Open Office odt formats, where the plan-XML itself only contains references, and the objects are stored beside the XML file - bytestreams and XML are then packaged in a ZIP file.

any comments? @kduretec @hkulovits @christoph-becker @mksplg

[kraxner]

(-) repositories had to understand the new format too