gtk: Clippy warns about unsound `Send` implementation

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Nightly clippy raises the following warning:

```
warning: this implementation is unsound, as some fields in `WindowState` are `!Send`
--> druid-shell/src/backend/gtk/window.rs:1271:1
|
1271 | unsafe impl Send for WindowState {}
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: `#[warn(clippy::non_send_fields_in_send_ty)]` on by default
note: the type of field `window` is `!Send`
--> druid-shell/src/backend/gtk/window.rs:173:5
|
173 | window: ApplicationWindow,
| ^^^^^^^^^^^^^^^^^^^^^^^^^
= help: use a thread-safe type that implements `Send`
note: the type of field `drawing_area` is `!Send`
--> druid-shell/src/backend/gtk/window.rs:180:5
|
180 | drawing_area: DrawingArea,
| ^^^^^^^^^^^^^^^^^^^^^^^^^
= help: use a thread-safe type that implements `Send`
note: the type of field `surface` is `!Send`
--> druid-shell/src/backend/gtk/window.rs:191:5
|
191 | surface: RefCell>,
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
= help: use a thread-safe type that implements `Send`
note: the type of field `handler` is `!Send`
--> druid-shell/src/backend/gtk/window.rs:196:5
|
196 | pub(crate) handler: RefCell>,
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
= help: use a thread-safe type that implements `Send`
note: the type of field `deferred_queue` is `!Send`
--> druid-shell/src/backend/gtk/window.rs:201:5
|
201 | deferred_queue: RefCell>,
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
= help: use a thread-safe type that implements `Send`
note: the type of field `parent` is `!Send`
--> druid-shell/src/backend/gtk/window.rs:206:5
|
206 | parent: Option,
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
= help: use a thread-safe type that implements `Send`
= help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#non_send_fields_in_send_ty
```

Start by running nightly Clippy for the GTK backend and inspect the unsafe Send implementation at druid-shell/src/backend/gtk/window.rs:1271, along with the WindowState fields listed in the warning. Trace how WindowState is used across threads and determine a safe resolution. Done means the warning is addressed without introducing unsound cross-thread behavior.

Written by the indexing model from the issue text.