feature: User info in proc widget for Unix-based systems

[ClementTsang]

Description

A description of the change and what it does. If relevant, please provide screenshots of what results from the change:

Adds users into the process widget (for Unix-based systems). This shows only in non-grouped modes, similar to state. Search is also supported.

In addition, a quick fix to prevent users from being in grouped mode when they tried to enter tree mode while grouped.

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On a side note, this change sorely highlights the need for a refactoring of how we do platform-specific implementations... this was a PITA to implement with proper OS family gating and lead to some awful code.

Issue

If applicable, what issue does this address?

Closes: #236

Type of change

Remove the irrelevant ones:

• New feature (non-breaking change which adds functionality)

Test methodology

If relevant, please state how this was tested:

Furthermore, please tick which platforms this change was tested on:

• Windows
• macOS
• Linux

If relevant, all of these platforms should be tested.

Checklist

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• Change has been tested to work, and does not cause new breakage unless intended
• Code has been self-reviewed
• Documentation has been added/updated if needed (README, help menu, etc.)
• Passes CI pipeline (clippy check and cargo test check)
• Areas your change affects have been linted using rustfmt (cargo fmt)
• No merge conflicts arise from the change

Other information

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