Customize location of the conversation list and conversation view

the default (and only) option is to have the list of conversations on the left and the preview on the right. Although you can change the size of the columns, this is not as useful as being able to have a full-width preview all the time without hiding conversation list.

It would be good to have an option to move the preview pane to the bottom so it is split horizontally instead of vertically like other email clients.