Panel resize handles is incompatible with assistive technologies

Describe the bug

Related to #8474. egui currently does not properly expose handles to allow users to adjust resize panels.

The following recording is on macOS VoiceOver. Notice that the contents of the handles is empty.

https://github.com/user-attachments/assets/71a4f7a9-8392-47d3-8b1d-d29873d8aa53

To Reproduce

1. Run the egui demo app and enable an assistive technologies, like VoiceOver.
2. Select a resize handle.
3. Notice that nothing is annouced and entering it gives "content is empty." Trying to resize does not work either.

Tested on

• OS: macOS Tahoe 26.6.2
• egui: 65e7db3c06d779c60ac56647bdd3011ed8ba1cbd

Expected behavior Like https://github.com/anhosh/egui_dock/pull/345, recording attached below:

https://github.com/user-attachments/assets/8be8e211-8d87-45c3-a7cb-cb88a07105b0

Additional context

https://github.com/emilk/egui/issues/8474#issue-5298643003 has a workaround for separators used in egui_dock.