Improve proc tree expand/collapse functionality

[nyurik]

Description

• On click, continue to toggle tree branch expand/collapse
• On Left and -, always collapse
• On Right and +, always expand

Also, a minor clippy fix to pass the CI

Issue

Closes: #1304

Testing

Manual testing as the change is fairly trivial. Let me know if anything else is needed.

If this is a code change, please also indicate which platforms were tested:

• Windows
• macOS
• Linux

Checklist

If relevant, ensure the following have been met:

• Areas your change affects have been linted using rustfmt (cargo fmt)
• The change has been tested and doesn't appear to cause any unintended breakage
• Documentation has been added/updated if needed (README.md, help menu, doc pages, etc.)
• The pull request passes the provided CI pipeline
• There are no merge conflicts
• If relevant, new tests were added (don't worry too much about coverage)

[codecov]

Codecov Report

Attention: 20 lines in your changes are missing coverage. Please review.

Comparison is base (4174012) 31.64% compared to head (265597d) 31.61%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1306      +/-   ##
==========================================
- Coverage   31.64%   31.61%   -0.04%     
==========================================
  Files          98       98              
  Lines       16644    16660      +16     
==========================================
  Hits         5267     5267              
- Misses      11377    11393      +16     

Flag	Coverage Δ
macos-12	33.33% <0.00%> (-0.04%)	⬇️
ubuntu-latest	33.17% <0.00%> (-0.04%)	⬇️
windows-2019	33.59% <0.00%> (-0.04%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
src/constants.rs	98.81% <ø> (ø)
...c/app/data_harvester/disks/unix/linux/partition.rs	0.00% <0.00%> (ø)
src/widgets/process_table.rs	67.20% <0.00%> (-0.31%)	⬇️
src/app.rs	1.48% <0.00%> (-0.01%)	⬇️

... and 4 files with indirect coverage changes

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