Add toolbar button to toggle legend visibility in EngDiff UI fitting tab

[RichardWaiteSTFC]

Is your feature request related to a problem? Please describe. Workspaces in the EngDiff UI have long names and the legend in the fitting tab obscures the data.

Describe the solution you'd like Add toolbar button to toggle legend visibility in EngDiff UI fitting tab

Describe alternatives you've considered Changing the workspace names to be shorter, but even if they were it would still be good to be able to hide the legend as the plot can get very small.

Additional context