Use ADSObserver in ContourPreviewPlot widget

[robertapplin]

Description of work

This PR ensures the ContourPreviewPlot widget uses the ADSObserver class to observe changes in the ADS. This makes the code simpler in the ContourPreviewPlot, and also helps avoid a bug as the ADSObserver class will make sure the previewplot is not subscribed to ADS notifications more than once. This has caused a crash in the past for other similar widgets: #37074

To test:

I have not found a crash for this widget. I think because the watchADS function is not called on the ContourPreviewPlot from outside the widget. Just do a simple test to make sure Inelastic S(Q,w) is still working:

1. Open Inelastic Data Manipulations
2. Go to S(Q,w) tab
3. Load irs26176_graphite002_red file from sample data
4. Click Run. It should run without a crash or an error

This does not require release notes because there is no user-facing change

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[jhaigh0]

Nice simplifying change