Add WaitCursor to some slow tasks in the GUI

[frode-aarstad]

Issue
Resolves #6025

Approach
Short description of the approach

(Screenshot of new behavior in GUI if applicable)

Pre review checklist

• Read through the code changes carefully after finishing work
• Make sure tests pass locally (after every commit!)
• Prepare changes in small commits for more convenient review (optional)
• PR title captures the intent of the changes, and is fitting for release notes.
• Updated documentation
• Ensured that unit tests are added for all new behavior (See
  Ground Rules),
  and changes to existing code have good test coverage.

Pre merge checklist

• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.

[codecov-commenter]

Codecov Report

Merging #6131 (fef3087) into main (526c565) will increase coverage by 16.84%.
Report is 27 commits behind head on main.
The diff coverage is 75.00%.

❗ Current head fef3087 differs from pull request most recent head 2a8182a. Consider uploading reports for the commit 2a8182a to get more accurate results

@@             Coverage Diff             @@
##             main    #6131       +/-   ##
===========================================
+ Coverage   65.62%   82.47%   +16.84%     
===========================================
  Files         349      350        +1     
  Lines       21495    21453       -42     
  Branches      817      834       +17     
===========================================
+ Hits        14107    17693     +3586     
+ Misses       7092     3462     -3630     
- Partials      296      298        +2     

Files Changed	Coverage Δ
src/ert/gui/tools/plot/plot_window.py	85.46% <66.66%> (+64.28%)	⬆️
src/ert/gui/simulation/simulation_panel.py	96.55% <100.00%> (+74.62%)	⬆️

... and 143 files with indirect coverage changes

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