Added None as a line option

[SilkeSchomann]

Description of work:

Added a new line style, "None". Here, only the error bars are plotted and not the line.

To test:

1. Load a workspace into MSlice
2. Do a cut
3. Click on the settings icon and change the line style to 'None'
4. Observe that the cut only shows error bars and no line
5. Change the line style to any of the other options
6. Check that the line on the plot is shown with the corresponding line style

Fixes #981

[adriazalvarez]

Following test instructions, now there's the option to plot None and then it only plots the scatter plot and error bars.
I'm happy to approve.

Additionally, maybe I have missed the option, but it seemed you have to change line options individually for each line represented in the plot. It would be nice future feature to have the choice to change all the line styles at once.

[SilkeSchomann]

Following test instructions, now there's the option to plot None and then it only plots the scatter plot and error bars. I'm happy to approve.

Additionally, maybe I have missed the option, but it seemed you have to change line options individually for each line represented in the plot. It would be nice future feature to have the choice to change all the line styles at once.

@adriazalvarez This is an excellent suggestion. I will raise an issue for this.