Add Repeat command

[mattstermiller]

A repeat command . will either repeat or retry certain actions.

• When an item is renamed and text was only prepended or appended to the existing name or full name, the repeat action will become prepending or appending that same text to other items.
• To augment #70, when a file or folder move or copy fails with an error, the repeat action would be to retry the move or copy. This should work on a per-item basis; for instance, if a move of a folder with 10 items failed because 1 file could not be accessed, the repeat would only retry that 1 failed file. If any failures succeeded, the previous undo operation should be updated.
• To complement #131, when a put or delete operation is canceled, the repeat action should complete the operation. This should include updating the existing undo operation with the full scope of the operation rather than appending another undo operation.
• Any subsequent action will replace or clear the repeat action.

The Repeat action should be shown in the status bar, docked to the right but left of the yank register.