Sets 'Stack Plots' for slicers as a global user preference

[jellybean2004]

Description

As suggested by @krzywon in PR #3806, the 'Stack Plots' checkbox has been added as a global user preference. Also adds this to the preferences documentation.

Fixes #3830

How Has This Been Tested?

Manually tested functionality.

Review Checklist:

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

Documentation (check at least one)

• 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)

[Copilot]

Pull request overview

This pull request adds a global user preference for the "Stack Plots" functionality when using slicers in 2D plots. Previously, this was hardcoded as True in the Plotter2D class; now it can be configured by users through the Preferences panel.

Key Changes:

• Added STACK_PLOTS configuration parameter with a default value of True
• Integrated the preference into the Plotting Preferences UI panel
• Updated the preferences documentation to describe the new setting

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/sas/system/config/config.py	Adds STACK_PLOTS configuration parameter with default value True and descriptive comments
src/sas/qtgui/Utilities/Preferences/PlottingPreferencesWidget.py	Adds checkbox widget for stack plots preference, includes signal handling and config restoration
src/sas/qtgui/Plotting/Plotter2D.py	Changes stackplots initialization from hardcoded True to use config.STACK_PLOTS
src/sas/qtgui/MainWindow/media/preferences_help.rst	Documents the new preference option with description of its behavior

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[krzywon]

Overall, the code looks good. Haven't tested functionality.

[krzywon]

If there is documentation on the plot stacking, a link to it from here would be helpful.

[JoGaudet]

I tested the functionality and it seems to work as intended. We can now see the option to click "Stack plots when using slicers", which automatically allow (or disallowed) the stacking of multiple slicers into the same plot. I tried the global preference and it works as intended for all slicers available.