[FTS-UI] Index defintion "preview" change detection

On selecting to edit an existing FTS index, the "Update Index" in the classic editor and the easy editor are both grey-ed out (or disabled) until a change in the definition is detected.

This change detection is supported by the UpdatePreview() method which runs in a second's interval by polling on all the index attributes to generate an index definition preview.

This entire process to detecting a change to index definition (by comparing the original definition against the polled preview) should be updated to be event driven as opposed to repeatedly polling for any changes.