A scene that makes a parent, nests two collections in it by drag and by
the menu, files frames into a child and opens the parent to see it count
both; stills of the tree and of the menu. The collections recording is
re-made now that the bitmap under the cursor is the photograph.
drive.py grows a multi-leg drag: a diagonal with much vertical in it is
taken by the grid's Flickable as a scroll before the DragArea can claim
it, so a drag to the sidebar goes sideways first.
