Split alignments track menu items into "Pileup" and "SNPCoverage" submenus

[cmdcolin]

This was sort of inspired by #1754 but does not fix it

I split the long list of track menu items for the combined snpcov+pileup alignments track into submenus, one for pileup, one for snpcoverage options

It also added a custom getter called "extraTrackMenuItems" instead of overriding composedTrackMenuItems in the SNPCoverageDisplay model because it was not possible to properly override composedTrackMenuItems and preserve the wiggle options

[cmdcolin]

This also removes SNPCoverage model's overriding of the renderProps function as the base version on LinearWiggleDisplay works ok for it

[codecov]

Codecov Report

Merging #1756 (82a8ab2) into master (4de4374) will decrease coverage by 0.01%.
The diff coverage is 59.57%.

@@            Coverage Diff             @@
##           master    #1756      +/-   ##
==========================================
- Coverage   58.80%   58.79%   -0.02%     
==========================================
  Files         454      454              
  Lines       21007    21018      +11     
  Branches     4975     4979       +4     
==========================================
+ Hits        12354    12357       +3     
- Misses       8343     8352       +9     
+ Partials      310      309       -1     

Impacted Files	Coverage Δ
...nments/src/LinearAlignmentsDisplay/models/model.ts	67.60% <ø> (ø)
...lugins/alignments/src/LinearPileupDisplay/model.ts	69.02% <ø> (ø)
...nts/src/SNPCoverageRenderer/SNPCoverageRenderer.ts	91.25% <12.50%> (-8.75%)	⬇️
...ins/wiggle/src/LinearWiggleDisplay/models/model.ts	72.38% <20.00%> (+0.88%)	⬆️
...ments/src/LinearSNPCoverageDisplay/models/model.ts	69.23% <76.47%> (ø)
...inearGenomeView/components/RefNameAutocomplete.tsx	89.36% <0.00%> (-4.26%)	⬇️
packages/core/util/index.ts	82.20% <0.00%> (ø)

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