Support `dwi` file format

[Anders429]

Song structs should be readable from and writable to the dwi format.

dwi predates sm, and is still supported by Stepmania. Format specification is available here: http://dwi.ddruk.com/readme.php

There is also a largely-incomplete write-up about the format on the Stepmania wiki: https://github.com/stepmania/stepmania/wiki/dwi

Required Tags:

• [ ] #TITLE
• [ ] #ARTIST
• [ ] #GAP
• [ ] #BPM

Optional tags:

• [ ] #DISPLAYTITLE
• [ ] #DISPLAYARTIST
• [ ] #DISPLAYBPM
• [ ] #FILE
• [ ] #MD5
• [ ] #FREEZE
• [ ] #CHANGEBPM
• [ ] #STATUS
• [ ] #GENRE
• [ ] #CDTITLE
• [ ] #SAMPLESTART
• [ ] #SAMPLELENGTH
• [ ] #RANDSEED
• [ ] #RANDSTART
• [ ] #RANDFOLDER
• [ ] #RANDLIST
• [ ] #SINGLE
• [ ] #DOUBLE
• [ ] #COUPLE
• [ ] #SOLO
• [ ] #BACKGROUND
• [ ] #END