list-windows: add --sort option

[waj]

Introduce a new --sort flag for the list-windows command. Supported values are recent, app-name, and window-title, and multiple values may be specified as a comma-separated list.

For backward compatibility, the default sort order is app-name,window-title. Using --sort recent sorts windows by most recently focused.

Although there was no comments in the discussion, this was motivated by #1890.

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