Drag rules from left panel into rewrites

[akissinger]

This would speed things up a lot.

[hmillerbakewell]

There is now an "Add to current derivation" option when right-clicking rules on the FileTree (provided you have a derivation open.)

Remember you can also search for rules in the "Add Rule" dialogue!

Drag-and-drop seems possible, but I don't want to target it in this release.