List only the displayed columns in "Manage table columns"

[jescalada]

The Params app becomes very slow with thousands of metrics. Specifically, the Table seems to have problems when trying to load and sort thousands of metrics at a time. One way to improve performance is to hide non-selected metrics and params by default: G-Research/fasttrackml-ui-aim#44

After these table enhancements, the related Manage Columns popover still has performance issues, taking many seconds to even open. We would like to virtualize the list to make sure table columns can be easily hidden or cherry-picked.