Visual plugin: SPHINCS+ fixes

[simlei]

Original issue: https://github.com/jcryptool/crypto/issues/287#issuecomment-605438817

• Prio Medium/high: visualizations are broken under Linux and would confuse the user.

• Medium: context help

• The visualizations on linux are broken, s. I1). Sphincs without the plus seems to not have these problems with the same kind of visualization. Fix: remove thread and replace it with suitable SWT idioms for tracking scrolling, dragging

• text field below description in I2) has no content, I am not sure what it does.

• see: https://github.com/jcryptool/crypto/issues/258

• see: https://github.com/jcryptool/crypto/issues/270

Textual:

• Online Help:
  • below the first "remark" there is a solitary dot. remove.

I1)

I2)

[lvcabral]

This was already implemented in v0.16.0 closing