Move message list column selector menu above the scrollbar.

[lemaitre54]

This is a small tweak, but I think it would be really nice to have back.

In previous versions, the message list column selector was stacked (mostly) above the scrollbar.

In the new version, the column selector has shifted to its own column, even though it never has content.

It would save a lot of space to stack them again.

[Jon]

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