FR: add option to fetch GTF from UCSC for custom workflow stepper

Currently, when you use the custom workflow stepper, the second step allows picking UCSC browser tracks to include in the package, which is nice. What is less nice is that when I select the VEuPathDB gene track, I get the bigbed file imported which isn't helpful when I need GTF (well, yes, I could convert it, but since the GTF is *right there*, it would be nice if I could just select fetching it).

Start by locating the custom workflow stepper and its second step, then trace how UCSC browser tracks are selected and imported. The change is complete when the VEuPathDB gene track can offer its GTF for fetching, rather than only importing the bigBed file, with the existing custom workflow behavior preserved.

Written by the indexing model from the issue text.