silx.gui.plot.iterms: Fixed Marker.setSymbolSize

[t20100]

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This is a rebase and rework of "Draft: Size for markers" #4026

Description from PR #4026:

Closes #4024

This API was exposed but not propagated the the backend.

Notice that the default marker size was not the same in mpl and opengl. That's something which have to be checked.

Changelog:

Markers now supports setSymbolSize