Register file hover and drop events for areas

A convenient feature would be to only allow dropping files in a specific region in the window. As I understand it, this is currently not possible as the pointer position is `None` while hovering and right after dropping a file. Maybe this is a winit limitation though.

**Describe the solution you'd like**

Having access to the pointer position while hovering would be enough. Even nicer would be to store the hovered and dropped files in an area's response.

**Describe alternatives you've considered**

Keeping track of which files have been hovered right before releasing, but this involves keeping track of the information over multiple frames and quickly becomes messy.

Start by tracing egui's file-hover and file-drop event handling, including the winit integration and area response APIs mentioned in the issue. Determine whether pointer positions and hovered or dropped files can be exposed per area; done means a specific region can accept or identify file drops without tracking state across frames.

Written by the indexing model from the issue text.