Fitting Window: Add Spectrum Line Plot Widget

[JackEAllen]

The redesign of the Spectrum Viewer Window will likely involve a few different iterations as this area of Mantid Imaging is actively being developed until the point where a Bragg edge offering have matured as a feature.
Due to this volatility to the GUI, we should place components into Widgets where possible and sensible to do so to make it easy to re-use where applicable throughout other sections of the Spectrum Viewer in the future if needed and also allow for easier relocation of existing components.

Create/add a widget to display the spectrum line plot for a given ROI in the Fitting Window.
Note that implementing the ROI visible in the below screenshot will be completed as part of a separate issue.

Spectrum Line Plot:

Fitting Window: