Drag and drop functionality

### What is the problem or limitation you are having?

I've built a Streamlit webapp for my girlfriend's business about 3 years ago and would love to rebuild it with native UI components that Toga provides so that she doesn't have to launch a powershell script that launches the Python webapp in the browser.

But for this I do need a way to drag files into the application and be able to do something with the filenames/paths & contents.

Does Toga already support this in some way and if not, I would love to request this as a feature.

### Describe the solution you'd like

Drag and drop functionality within a Toga application

### Describe alternatives you've considered

Other frameworks, or using Tkinter as a fallback which doesn't look great.

### Additional context

_No response_

Start by checking whether Toga already provides drag-and-drop support within a native application. Define what the application should receive from dropped files—filenames or paths, plus contents—and how that should work across supported native platforms. Done means a Toga application can accept dragged files and access the requested file data.

Written by the indexing model from the issue text.