Expose `.move_to_top` for `Window`

**Is your feature request related to a problem? Please describe.**
I'm drawing multiple windows which expand their contents on hover (screenshot). The problem is in setting which window is rendered on top.


I tried changing the order in which `window.show(...)` is called but it seems as long as they have the same IDs their z-order remains the same.

I also tried using `ui.with_layer_id` but this doesn't work for the windows themselves, as in

```rust
Window::new(...).show(|ui| {
ui.with_layer_id(...);
})
```

What I noticed happens is clicking on a window puts it in front of all other windows. I tried using `ui.request_focus()` but this also doesn't work.

The only solution I found digging into egui's source code is

```rust
let window = Window::new(...);

ctx.memory().areas.move_to_top(window.area.layer());

window.show(ctx, |ui| { ... });
```

but the problem is `memory().areas` and `window.area` are both private fields. I did this by just making them public with a crate override, but there probably is a reason for them being not directly accessible.

**Describe the solution you'd like**
Expose an API for `Window` that enables moving it to the front like `areas.move_to_top()` does.

**Describe alternatives you've considered**
I've considered simulating a click on the window, but couldn't find an easy way of doing this. It seems `.move_to_top()` is only called based on interaction events, but since in my case the UI actually responds to a click I would need to do something more complicated.

In my case clicking removes the item & window, so the only way to move it to the front is click & hold, move the mouse away, and release, which moves the window to the front without triggering click.

Start by reading Window::show and the existing ctx.memory().areas.move_to_top(window.area.layer()) path described in the issue, including how interaction events trigger it. Trace the relevant Window and area ownership boundaries, then define and verify a public Window-level API that brings the window to the front without requiring private fields.

Written by the indexing model from the issue text.