Duplicate up button

[CatsAreFluffy]

It'd be nice to have a button to make a copy of a statement above that statement. You can already do this using expression selection, but that's slow for complicated statements.

[CatsAreFluffy]

The existing duplication operation is similar to what I'm proposing, but that leaves the original justification on the top copy, whereas duplicate up would leave it on the bottom copy.

[expln]

I plan to replace the existing "duplicate" button with two new buttons - one to duplicate down, another to duplicate up.

[expln]

This is ready on dev.