Use the same screen for both single- and multi-file operations in the File Commander

[prl]

The file commanded has different screens for single- and multi-file operations.

A single screen should be able to handle both sorts of operation, as requested in feature request 324.

The screen should be a markable item screen as in File Commander, MEDIA.

Operations then apply to marked items, or if no marked items, to the item that currently has focus.

This interface is similar to the DP series file operations in the File Player.