Enhancing UI and background selection.  Refs #7652.

1. Add visibilities to properties used by specific functions of the
algorithm;
2. Add more support to fit Chebyshev background function.
Added default value to boundaries.  Refs #7652.
Added an option to read in a background function.  Refs #7652.
Merge branch 'feature/7652_process_bkgd' into develop