Make drag-and-drop target for Clipboard widget and Clipboard manager window

### Feature Description

Dropover (https://dropoverapp.com) style interactions for the clipboard widget icon on the bar and for dragging into the clipboard manager:

### Use Case

Instead of ping ponging back and forth between the file manager or browser and the target program like a web browser, discord or media creation apps (like resolve, reaper, krita etc when they support drag and drop on wayland) this lets you gather all items in one place from different directories or tabs over time then deposit all items at once without polluting temporary directories with saved files.

### Compositor(s)

All compositors

### If Other, please specify

_No response_

### Proposed Solution

https://dropoverapp.com/tips
1. Drop onto the menu bar widget: Drag files, selected text, images, or URIs onto the widget icon and release to add it to clipboard. Hold over the icon to open clipboard manager to create bundled item collections.
2. Open the clipboard history panel and drop there: Drag content, open manager with shortcut, then drop to add an item or create groups.
3. Watch downloads or specified directory for new items to quickly paste or drag item.

### Alternatives/Existing Solutions

https://github.com/mijorus/collector

No files, tests, or entry points are identified, so first map the clipboard widget, manager window, and drag-and-drop handling in the shell. Done would cover dropping files, text, images, and URIs onto the widget or manager, grouping items, and the proposed download-directory behavior across the listed compositors.

Written by the indexing model from the issue text.