Subset Edit Plugin: Implement live-updating

[pllim]

This is a follow-up of #1427 . If we decided live-updating is no longer needed, then we can just archive this.

@duytnguyendtn and @kecnry both wanted live-updates. Several different ways proposed: Up/down arrows, tab, enter, when lost focus.

@pllim prefers not to live-update at all unless we determine that performance will not be impacted when we stress-test the feature. See #1427 (comment)

@camipacifici said the following at #1427 (comment) :

• I would not want the live update to kick in when I start modifying the value. It would be nice to have it when I click the arrows, but it is useful really only for Imviz where a jump of "1" is ok. A jump of "1" in Specviz is completely useless because it goes to a completely different region of the spectrum (e.g. the starting value is 1.234um, I click the arrow, it goes to 2um), so I do not think I would ever use the arrows there.
• It would be nice to have the live update when tabbing to the next entry. Could this be an option?
• Said that, I do not find a big overhead in, for example, tabbing twice and then hitting return. I guess I need to try with different science cases, but a non-live update looks good to me (for now).

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