Allow creating alternative "add track workflows" from within the "Add track" widget

[cmdcolin]

This creates a concept of having different workflows inside the Add track widget.

This is helpful for adding track types that are sufficiently different from the normal add track workflow we have, such as creating a multibigwig by pasting a list of files

Example screenshot

fig 1. the multi-wiggle add track widget

fig 2. the select box showing the different add track widget options

I propose this as a new pluggable element type called "add track workflow" that consists basically of a ReactComponent. This component is displayed fully separately from the stepper in the normal workflow (does not tie into the stepper workflow)

[codecov]

Codecov Report

Merging #3102 (98d484e) into main (18290ec) will decrease coverage by 0.05%.
The diff coverage is 52.78%.

@@            Coverage Diff             @@
##             main    #3102      +/-   ##
==========================================
- Coverage   58.91%   58.85%   -0.06%     
==========================================
  Files         640      644       +4     
  Lines       28843    28897      +54     
  Branches     6999     7017      +18     
==========================================
+ Hits        16992    17008      +16     
- Misses      11469    11511      +42     
+ Partials      382      378       -4     

Impacted Files	Coverage Δ
...kages/core/BaseFeatureWidget/BaseFeatureDetail.tsx	73.14% <0.00%> (-0.43%)	⬇️
packages/core/util/index.ts	85.57% <0.00%> (-1.93%)	⬇️
...nts/src/SNPCoverageRenderer/SNPCoverageRenderer.ts	73.38% <ø> (ø)
plugins/bed/src/BedAdapter/BedAdapter.ts	85.71% <ø> (ø)
plugins/bed/src/BedTabixAdapter/BedTabixAdapter.ts	90.24% <ø> (ø)
plugins/bed/src/util.ts	88.31% <ø> (ø)
plugins/data-management/src/index.ts	90.00% <ø> (ø)
plugins/menus/src/AboutWidget/index.ts	100.00% <ø> (ø)
plugins/menus/src/HelpWidget/index.ts	100.00% <ø> (ø)
...rtSessionWidget/components/ImportSessionWidget.tsx	0.00% <0.00%> (ø)
... and 21 more

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[cmdcolin]

might go ahead with merge, can revert if the idea is too weird :)