[StatsWidget] optimizations

[t20100]

The StatsWidget can be improved to avoid useless computation:

• Disable update when widget is not visible
• Check plot items event type and only update stats when needed (i.e., data or transform changed)
• When only displaying selected item, keep a cache of stats and discard them when item changes
• Check that data copy are avoided by using copy=False when getting the data.

[t20100]

And consider asynchronous computation of the stats