ui: Add percentage to flat and cumulative columns in table

[brancz]

This is a start of #3880, but we should separate it out to separate columns instead as the line breaks are not very good this way.

@metalmatze had some WIP work that allowed dynamically adding columns, I think that would be the right thing to do here.

[alwaysmeticulous]

✅ Meticulous spotted zero visual differences across 170 screens tested: view results.

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[metalmatze]

It would be great if we can merge #3780 first.

[brancz]

Agreed, like I said in the description I’d want this to use the column selector anyway.

[monicawoj]

As the work was rather related, I've gone ahead and added these percentage columns directly in #3780 - please take a look there whenever you get a chance.

[brancz]

Superseded by #3780