Fix alarm tree selection preservations

[shroffk]

Addresses #3652

The selection in the alarm tree is cached and then restored.

Checklist

• Testing:

  • The feature has automated tests
  • Tests were run
  • If not, explain how you tested your changes

• Documentation:

  • The feature is documented
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  • Release notes:
    • Added an entry if the change is breaking or significant
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[shroffk]

@kasemir could you give this a quick look... nothing particularly complicated.
I cache the current selection, if the "update" is touching one of the cached selection then I replace them with the new one, and then restore the selection after the update even processing is complete.