#146 Loading and saving .S (from .d40 image) snapshot files

[Arki55]

I have implemented loading and saving .S snapshot files, which are being saved by the SNAP button on Didaktik D40's floppy drive.

See my implementation here:
https://sourceforge.net/u/arki55/tickets/1/

Implement support for ".S" snapshots from Didaktik D40's floppy drive.
There is already support for this when reading/writing from/to the emulated FDD,
however I want to support these files when they are out of the floppy image.
I might be able to implement both reading and writing.

Helping materials / books :
- "Komentovany vypis MDOSu" by Kvaksoft from 1995.
- "Rutiny ROM D40" by Proxima from 1993

Recommended extension : ".S" / ".s"
(it has to be as .S in MDOS floppy, otherwise file will not be found)

Please state your objections if this extension is not appropriate.

Branch with library changes:
https://sourceforge.net/u/arki55/libspectrum/ci/features/d40-snapshots/~/tree/

Branch with app's changes:
https://sourceforge.net/u/arki55/fuse/ci/features/d40-snapshots-app/~/tree/

[Arki55]

Pull requests:

Library:
https://sourceforge.net/p/fuse-emulator/libspectrum/merge-requests/6/

App:
https://sourceforge.net/p/fuse-emulator/fuse/merge-requests/12/

[Arki55]

See also
https://sourceforge.net/p/arki55-fuse-mod/tickets/1/