Add subheading for label sections

What does this MR do and why?

When adding and removing label filters we show the labels that are applied, that are to be applied and that are to be removed when "Apply" button is clicked. This MR adds small subheading that more clearly show the state of each of the labels.

Why?

I thought it was important to explicitly explain to the user the state of the filter because it took me a long time to understand why the labels were listed there and it was unclear to me why some labels have the color darkened and why some have a close button and others do not. I think these heading better communicate this information. This clearer explicit communication also makes the filter more accessible.

References

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Before	After

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Related to #519777 (closed)