Preview tab: retain ROI selections on loading run

[rbauststfc]

Is your feature request related to a problem? Please describe.

On the Preview tab of the ISIS Reflectometry GUI you can select regions from both the instrument view and slice viewer plots. If you then load a new run these regions are cleared.

Describe the solution you'd like

Our scientists would prefer for any selected regions to remain when a new run is loaded. This wasn't requested, but we should presumably provide an easy way of clearing all regions for the case where the scientists do want to start from the beginning.