On-the-fly updating of integration region preview in DetectorScene

Resolves #246 (closed)

Added spin boxes for peak end, background begin and background in to PeakViewWidget, connected to the visualisation of integration regions in DetectorScene. When these values are adjusted, the visual of the integration region changes on-the-fly, allowing the user to see what the new integration region looks like before integrating. These spin boxes are linked to the integration spin boxes, such that they both display the same value.