Add 'undo' menu item to undo the single latest copy operation (Issue #819)

First of four steps for full undo/redo support.

- This first commit will only bring support for 'copy'. Support for other operations will follow soon.
- Undo will be provided in the 'edit' menu (or via STRG+Z shortcut).
- 'ThunarJobOperation' models a single user operation, containing one to many source- and target-files.
- A 'ThunarJobOperation' can be inverted to do the opposite of what was originally done.

MR !266