Store Seq.JSON in addition to SeqN.txt when sequence is saved

[joswig]

• When saving a sequence save both formats
• On opening said sequence the seqN.txt format will be preferred since it contains anything that may not fit in seq.json (whitespace and errors). When no seqN.txt version exists, the seq.json format will be converted upon opening

[dandelany]

Closing per @goetzrrGit