Feature request: Add "none" option to line style

[mducle]

Original reporter: ISIS / M. Aouane

Sometimes users do not necessarily want to plot a line spectra - just the points or errorbars. MSlice allows users to customize the plot using the "Cogs" (Settings) icon which brings up another window. In this window, on the right hand side is a box which contains all traces in the plot with options to change the Color, Style etc - for line Style a user requests that there be an addition option None which will just plot the markers and not the line (the Show Line check box governs whether the whole trace (line and marker) is shown or not).