Fitting widget refactor part3

[rozyczko]

Description

Refactored out:

• widget logic
• fitting logic to FittingControler
• constraint logic to ConstraintManager

Shaved off over 400 lines.

How Has This Been Tested?

Local win10 runs

Review Checklist:

[if using the editor, use [x] in place of [ ] to check a box]

Documentation (check at least one)

• [ X] There is nothing that needs documenting
• Documentation changes are in this PR
• There is an issue open for the documentation (link?)

Installers

• There is a chance this will affect the installers, if so
  • Windows installer (GH artifact) has been tested (installed and worked)
  • MacOSX installer (GH artifact) has been tested (installed and worked)
  • Wheels installer (GH artifact) has been tested (installed and worked)

Licensing (untick if necessary)

• The introduced changes comply with SasView license (BSD 3-Clause)