add the ability to split an export file into the contained files

[PeterPawn]

• extra applet and an additional option for 'decode_export'
• consider the reverse processing (build export file - with customizable header lines - from single files) as another applet

[PeterPawn]

split file done with extra applet in 6298a6a

[PeterPawn]

reverse processing delayed - consider it again later, for a next version