I want to add a new api for XdgShellHandle to know then the size of toplevel changed

https://github.com/Smithay/smithay/blob/master/src/wayland/shell/xdg/handlers/surface.rs#L254-L262

I want to know that when it is changed, Then I can do resize later or any other thing. Can this pr be accept.

for example, add
```rust
fn  geometry_changed(&self, x: i32, y: i32, width: i32, height: i32) {
}
```

and set a default function, which is empty