Fixes unique ID generation

[jellybean2004]

Description

For multiple slicers, the slicer ID on the list in the slicer parameters window did not match that on the slicer plot. This was because the ID on the plot was adding 1 to the actual slicer number.

This PR fixes that bug by correctly generating IDs for the plots in line with the slicer parameters list.

How Has This Been Tested?

Manually ran SasView to check IDs.

Review Checklist:

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)