Frameless Mode has some minor problems.

Thanks for adding frameless mode. It is what I need in order to use my own custom title bar. Unfortunately for that use case the window should also be resizeable. I tested some configurations on Windows 10 and only found the **EDGE (non-resizable)** version to provide the expected result. I used the following code for testing:
```
use web_view::*;

fn main() {
web_view::builder()
.content(Content::Html(HTML))
.size(150, 150)
.frameless(true)
.debug(true)
.resizable(false)
.user_data("")
.invoke_handler(|webview, arg| {
match arg {
"exit" => webview.exit(),
_ => (),
}
Ok(())
})
.run()
.unwrap();
}

const HTML: &str = r#"

exit

"#;
```

These are the results: