Fixes Invariant plot sliders

[jellybean2004]

Description

After the invariant refactor, the in-plot sliders were rendered non-functional, serving only as a visual guide. They were still movable, but reset to the extrapolation position based on the text field/in-window slider.

This PR converts the sliders into non-movable lines by updating the property of QRangeSliders. Some more work was done to optimise the performance by only updating the sliders when needed instead of the entire plot, which involved slight refactoring of the invariant plotting logic.

Fixes #3826

How Has This Been Tested?

Manually tested + pytest

Review Checklist:

Documentation

• 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

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

[jellybean2004]

The ubuntu tests were failing. I managed to fix this with the changes in commit 4c780d5

[DrPaulSharp]

This looks good, I'd just like you to take a look at entering values in the comboboxes for extrapolation. I've found that this has been made less responsive as a result of these changes, can you find a way to restore the performance here?

[jellybean2004]

This looks good, I'd just like you to take a look at entering values in the comboboxes for extrapolation. I've found that this has been made less responsive as a result of these changes, can you find a way to restore the performance here?

Hi @DrPaulSharp. Thanks for your feedback.

I have added a commit trying to restore the performance for the text fields, which includes a slight re-jigging and cleaning up unnecessary calls.

I did notice a slight delay between the pop-up appearing and its content loading. I could not figure out a way to curb this. Any thoughts and suggestions are welcome.

[jellybean2004]

Probably best to put it in the next bug-fix release instead of 6.2.0, as this is not a critical issue.