[6.x] Change how the slot with the "customize columns" works. Instead of an…

[jaygeorge]

Description of the Problem

Currently, the "Customize Columns" button is absolutely positioned to the right.
It's positioned absolutely in case there are many custom filters, which would otherwise push the "Customize Columns" button off screen, like in this example, where the numerous filters overflow underneath the control:

This is OK, but it's a fragile solution when we want to pin more buttons in the same space as "Customize columns".
I found that extra buttons now clash with the filter overflow, like this:

What this PR Does

Makes the layout less hacky by positioning the "Customize Columns" button within the parent flexbox layout

(After) you can see the filters now fade out as they overflow in the flexbox layout, rather than flowing underneath the buttons to the right

If we inspect the DOM we can now see the right icons are part of the parent flex layout, including a gap

How to Reproduce

1. Go to a collection page
2. Add a load of filters so they start to overflow below the Customize Columns button