Use ADSObserver for PreviewPlot widget

[robertapplin]

Description of work

This PR ensures the PreviewPlot widget uses the AnalysisDataServiceObserver to keep a track of events happening in the ADS. It is better to use this observer than re-implement the Poco observers. There was a bug, fixed by #37074 , which would not have happened if we had used AnalysisDataServiceObserver.

To test:

1. Go to 'Inelastic'->Data Manipulations
2. Load data into the Symmetrise Tab
3. Click Preview, then click Run
4. Close the interface
5. Open the Data Analysis interface
6. On MSD Fit load data
7. Select Gaus function
8. Click Run. There should be no crash

This does not require release notes because it is internal maintenance

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[adriazalvarez]

Thanks for this update to the previous fix. It looks more succinct and neat now, using the ADSobserver class, and the crash remains fixed.
Looks good to me, just added couple of comments.

[adriazalvarez]

I guess is up to your personal preference, in some other places of mantid this is written with UNUSED_ARG(..) macro. Would you prefer this rather than the macro?

[robertapplin]

Yeah good question. I guess UNUSED_ARG is more explicit as its clearer what its doing, but then (void)var; is probably easier to remember as it doesn't require a named macro. Either way I think it would just be down to preference, but I'm not sure I have a preference!

[adriazalvarez]

Ok. No problem.

[adriazalvarez]

Further improves on stabilizing the use of interfaces across indirect/inelastic categories.
Happy to approve it.