[jaygeorge]

Depends on #15677.

Description of the Problem

#15677 lets you drag a whole selection at once, but while you're dragging, nothing tells you how many assets are coming along. The browser's drag image only shows the one you grabbed, semi-transparent.

Dragging again while an earlier drop is still in flight could also pick up assets from that first move. Their IDs are about to change, so sending the old ones would fail the whole move.

What this PR Does

• When you drag more than one asset, a count badge follows the cursor. It replaces the browser's own drag image, because browsers always draw those semi-transparent and there's no way to turn that off. It sits just below and to the right of the cursor, like Finder, so it doesn't cover whatever you're dropping onto. Dragging a single asset keeps the normal drag image.
• Selected assets that are still moving from an earlier drop are left out of the drag. The asset you grab always comes along.

drag-whole-selection.mp4

How to Reproduce

1. Go to Assets and select a few assets.
2. Drag one of them. A badge with the count follows the cursor.
3. Drop them onto a folder or a breadcrumb, then drag the rest of the selection somewhere else before the first move finishes. Only the assets that aren't already moving come along.