silx.gui.plot.PlotWidget: Issue with signals passing the "legend" rather than the item

[t20100]

Since item legend is no longer unique, sigActive[Curve|Image...]Changed signals have troubles when changing active item between 2 different items with the same name, since what the signal emits is the name of the item and the active item has already changed.

In this case there is no way to know which item was previously active.