[silx view]NXdata defaults

[vasole]

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@woutdenolf

Since silx view can only visualize one NXdata and for the time being we are not specifying the layout of the plot using a silx specific field, it would be convenient to have a singleton containing the the last user choice made for the type of plot.

Example: the user has interactively selected to have a solid representation of a scatter plot for the NXdata of a certain scan. If he chooses a different NXdata scatter plot the choice is lost even when coming back to the same scan.

[vasole]

Related to #2100

[t20100]

I don't know for the NXData widget, but for (some/most?) other views in silx view, the visualization settings are kept when changing the viewed dataset. Is it what you want? To me it seems a good way to go, but I'm not sure it is done for all views.