Misc refactors to FileSelector

[cmdcolin]

This is a small refactor to fileselector to try to make the code a little more compact

• Uses a useState for anchorEl instead of useRef, a pattern seen in mui docs pretty often
• Shorter variable names, just improves code density and reading comprehension a bit (IMO)
• Removes the hierarchy of Popper-> Grow->Paper->ClickAwayListener->Menu, instead just having Menu which already has effects on it

[codecov]

Codecov Report

Merging #2468 (7087d59) into main (473b37b) will increase coverage by 0.00%.
The diff coverage is 39.39%.

@@           Coverage Diff           @@
##             main    #2468   +/-   ##
=======================================
  Coverage   61.15%   61.16%           
=======================================
  Files         539      539           
  Lines       25072    25062   -10     
  Branches     5858     5855    -3     
=======================================
- Hits        15332    15328    -4     
+ Misses       9422     9416    -6     
  Partials      318      318           

Impacted Files	Coverage Δ
packages/core/ui/FileSelector/FileSelector.tsx	58.46% <39.39%> (+2.46%)	⬆️
products/jbrowse-web/src/util.ts	27.27% <0.00%> (ø)

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[cmdcolin]

@peterkxie said the dropdown example comes from https://mui.com/components/button-group/#split-button but it should be fine to remove the wrapping elements