Use RegionSelector widget in Sqw tab

[robertapplin]

Description We want to use the RegionSelector widget (i.e. the SliceViewer) seen in the Preview tab of the Reflectometry GUI inside the Sqw tab of the Data Manipulations GUI. It provides much better interactivity than the current ContourPreviewPlot we use in the Sqw tab. It will also allow users to select a region for their data using a rectangle draw tool, rather than having to manually specify values inside of 4 different spin boxes.

Implementation plan The plan is to create a new View for the Sqw tab. This will have the same interface (functions) as the current view, and so it can be easily switched in and out of the current presenter while this new view is being developed. It will mean we do not need to switch to using the new view until we are satisfied it is fully working.

At this point, we can delete the old view. I think we can also remove the ContourPreviewPlot widget files as this is only used in the Sqw tab.

Acceptance criteria

• A new view has been created for the Sqw tab of Inelastic Data Manipulations, which uses the SliceViewer
• It is possible to draw a rectangle to select a range of Energy and Q values

[SpencerHowells]

@robertapplin Just one thing. In the the present range selection you caanot have values of Q or Energy where there is no data. The prime example was user starting Q from 0 so that first few spectra had no data and then complaing that fitting was crashing!