[plot] Add a widget to display Scatter legends

[loichuder]

So far, there is only CurvesLegendWidget. Nothing for Scatters, nor for images (#1461)

Best IMO is to add it in silx by extending (or refactoring while at it) CurveLegendsWidget to support Scatter items (which, from a quick look at it, doesn't look to complex)

https://gitlab.esrf.fr/workflow/ewoksapps/ewoks3dxrd/-/merge_requests/163#note_440102

[t20100]

Interest of images sounds more limited IMO, but it should not be too complex to provide minimum support for any item in the plot.

[loichuder]

First step in #4404