added crosshair option in mantid

[KyleQianliMa]

Description of work

This PR adds a crosshair toggle functionality in mantidplot.
EWM 5302

Summary of work

After loading a workspace in mantid. Right click on the workspace -> plot data. At the upper right corner there is a '+' option which enables a crosshair being drawn on the plot. Current color option of the crosshair is set to red to distinguish black background if one choose to do color contour plot.

Further detail of work

To test:

Load data into mantid workspace. Right click on the workspace -> plot. Toggle on the crosshair option in the upper right panel with an icon of '+'.

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[rboston628]

Works as expected. Where the crosshairs line up matches the printed mouse location. Good work.

[jhaigh0]

During smoke testing for release 6.12, we've found issues with the crosshair option.

• Works incorrectly for tiled plots (when hovering over any plot the crosshair only displayed in the lower-right plot.
• Shouldn't be enabled for 3D plots
• If you have the crosshair activated and change the normalisation (via the right-click menu) you get a crash. It also seems to block the axes rescaling in this case?

[sf1919]

This was not included in manual testing so the manual testing instructions need to be updated too.