Drag captions into place

Grab the captions in the preview and slide them up or down. Where you drop them is where they burn.

The position control still exists and still wins when you use it, because the renderer only knows three bands. Dragging past a band boundary moves the control with it, so the two can never disagree about where the text is.

Horizontal dragging is deliberately absent. The burn centres captions horizontally, and a control that let you move them sideways would promise something the export cannot do.