feature: add customizable process columns

[ClementTsang]

Description

A description of the change, what it does, and why it was made. If relevant (such as any change that modifies the UI), please provide screenshots of the changes:

Add the ability to customize the order and type of columns in the process widget. This is currently a global setting, and we ignore duplicates.

Will do documentation changes in a separate PR.

Issue

If applicable, what issue does this address?

Closes: #662

Testing

If relevant, please state how this was tested. All changes must be tested to work:

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

Patch coverage: 93.26% and project coverage change: +2.08 🎉

Comparison is base (c99e98c) 25.93% compared to head (46923fa) 28.02%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1115      +/-   ##
==========================================
+ Coverage   25.93%   28.02%   +2.08%     
==========================================
  Files          94       95       +1     
  Lines       15586    15923     +337     
==========================================
+ Hits         4042     4462     +420     
+ Misses      11544    11461      -83     

Flag	Coverage Δ
macos-12	29.50% <93.26%> (+2.19%)	⬆️
ubuntu-latest	29.44% <93.26%> (+2.17%)	⬆️
windows-2019	29.55% <93.26%> (+2.23%)	⬆️

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

Impacted Files	Coverage Δ
src/app.rs	0.14% <0.00%> (ø)
src/app/query.rs	0.00% <0.00%> (ø)
src/constants.rs	5.13% <ø> (ø)
src/widgets/process_table/proc_widget_column.rs	62.26% <78.57%> (+6.56%)	⬆️
src/options.rs	76.19% <80.95%> (+0.04%)	⬆️
src/widgets/process_table.rs	61.68% <95.93%> (+24.69%)	⬆️
src/options/process_columns.rs	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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