SPRITE import: remove "PLUS3" header

[NealeTools]

Description:
The SPRITE editor exports without headers, and these are imported fine in NextBASIC.
However, BANKs SAVED from NextBASIC always contain a 128 byte "Plus3" header, and are then subsequently imported incorrectly into the ZX SPRITE tools.

Repro:

1. Make SPRITE sheet online
2. Export.
3. Import into a BANK in NextBASIC.
4. SAVE BANK (file will report being 128 bytes larger than intended export size due to addition of header)
5. Import into ZX tools online.

Suggested resolution(s):
A. Check file size on import (step 5) and strip leading 128 bytes (or make it an option to do so?)
B. Check for presence of "PLUS3" code within the initial few bytes. If present, strip first 128 bytes on import.

Discussion here:
https://www.facebook.com/groups/ZXNextBasic/posts/1289249144934755/
and (older, original, with .extract solution)
https://www.facebook.com/groups/ZXNextBasic/posts/817479465445061/