Pan asset

[thepaperpilot]

Add a button that opens a window where you can move an asset in relation to its old position. That is, allow you to drag it and, when you press save, it moves all instances of that asset (using the assset's pan array, this part is already built in).
Maybe add some way of having a live preview - either in the editor, the stage, or the panning window itself.
Automatically open this window when you replace an asset image with a new one, since that's the primary use case for this feature.