Increase ease of fitting mslice cuts

[MialLewis]

At the moment if wanting to fit an mslice cut, you have to save the workspace to mantid, then open the plot there to use the browser.

It has been requested to be able to fit within the mslice cut plot.

This is probably not realistic, but we could provide a button that is active when slice is not standalone which when clicked exports the cut straight to the mantid plot window.

Requested by Adroja.