Manual: nested collections, and the ghost drawn as it should be

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.

docs/manual/README.md

@@ -74,6 +74,18 @@ photograph can be in several, and the badge on its cell counts them.
 
 ![Filing photographs in a collection by dragging them onto it](media/library-collections.gif)
 
+Collections nest. Drag one onto another to put it inside; `+` with a
+collection selected — or `New collection inside` from its menu — makes a
+child. A parent shows everything its children hold, and its count says so.
+Right-click a row (hold it, on a tablet) for the menu: rename, nest,
+move back to the top level, keep it offline, delete.
+
+![Making Trips, nesting Alps and New York inside it, and opening the parent](media/library-nesting.gif)
+
+![Trips showing both of its children's photographs](media/library-nesting.png)
+
+![The menu on a collection](media/library-collection-menu.png)
+
 ## Developing a photograph
 
 Click a thumbnail to open it. The column on the right is every adjustment;