Add persistent legend visibility toggle

[pbeaucage]

As discussed in #2215, adds a new instance variable to PlotterWidget, self.legendVisible which tracks the user's preference for having the legend visible/invisible (this is distinct from self.showLegend which tracks whether the plot has a legend at all). The onToggleLegend and similar methods are updated to toggle self.legendVisible then trigger a redraw rather than just modifying the existing plot. Tested and appears to work on Win10.

[krzywon]

I've noted one issue that needs to be fixed in a separate comment here.

Not surprisingly, on redraw, the location of the legend resets to the original position, but that is likely beyond the scope of this PR. I'm going to create a plotting discussion (if there isn't one already) about this. Plots are completely reconstructed on redraw, resetting many attributes.

[wpotrzebowski]

Works also on Mac as long as plot window is not closed (which should be as it is) and except bug @krzywon pointed out.

[wpotrzebowski]

Also looked at the other perspectives and it works for all except Corfunc, which has differemt plotting mechanism. The intention is to make it also available for Corfunc, however this is not something that will be done for this PR.

[krzywon]

The most recent change fixes the issue I noted earlier.