Allow operations on focused thread in single-pane list

The Problem

When browsing threads in a single-panel layout, to act (e.g. star, delete, reply) on a currently focused thread, you have to either open it first (enter), or select (x), which is an unnecessary extra keypress.

Proposed Solution

In a single-pane layout, when there are no threads x-selected, all commands should apply to a focused thread instead.