The Edge backend doesn't dispatch events when the window is being dragged around

When I drag the window around, my callbacks issued through the `Handle` don't get called.
This does **not** happen with the IE backend (without the "edge" feature).

From my very short debugging, the `PostThreadMessage` call inside webview-sys still returns `TRUE` even when this happens.

**Cargo.toml**
```toml
[dependencies]
web-view = { version = "0.7", features = ["edge"] }
```

**main.rs**
```rust
use std::time::Duration;
use web_view::*;

fn main() {
let html = r#"




function change_text(text) {
document.getElementById('mybutton').value = text;
}


"#;
builder()
.content(Content::Html(html))
.user_data(())
.invoke_handler(|webview, arg| match arg {
"button_click" => {
{
let webview = webview.handle();
std::thread::spawn(move || {
std::thread::sleep(Duration::from_secs(2));
webview.dispatch(|webview|
// if you are dragging the window around, this doesn't get invoked
webview.eval("change_text('Goodbye, world!')"))
});
}
Ok(())
}
_ => unimplemented!(),
})
.run()
.unwrap();
}
```
Affected versions:\
- 0.7.2
- master
- (possibly others)

Start by reproducing the issue with the Cargo.toml dependency and main.rs example, then inspect the webview-sys PostThreadMessage path used by Handle::dispatch. Done means callbacks still dispatch while the Edge window is being dragged, matching the IE backend behavior.

Written by the indexing model from the issue text.