Decouple r/w XML and datafield

Saving datafields is moved from writeTo to writeDataFiles.

Reading datafields is moved from readFrom to readDataFiles.

writeTo and readFrom methods are kept only for XML streams.

This resolves #493 (closed)

changed milestone to %20 GUI refactoring

added GUI simplify labels

requested review from @j.wuttke

assigned to @m.svechnikov

much better, thanks

approved this merge request

merged

mentioned in commit 742755ae