EMailBrowser: Fix packing of preview pane.
Using gtk_box_new() instead of gtk_vbox_new() changes the way child widgets are packed by default via gtk_container_add(). gtk_vbox_new() expands them by default, gtk_box_new() does not, and the preview pane was being packed with gtk_container_add(). Packing widgets into a GtkBox with gtk_container_add() is usually the wrong way. Use gtk_box_pack_start() instead, with expand=TRUE.
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