Refactor the Corfunc slider to be more general

[DrPaulSharp]

Following the merge of #3789, the slider used in Corfunc will also be used directly by the Invariant perspective. To avoid cross-perspective dependencies, and enable greater functionality for the slider, it should be moved out of Corfunc and refactored into a general slider - perhaps called the "extrapolation slider". One possible place for the slider to live is in qtgui/Utilities

[DrPaulSharp]

Once this is done, the slider in the Invariant perspective should be refactored such that the low-q end and high-q start cannot be moved beyond the extremes of the dataset.