Add a getter of main window dragging state

[HoKim98]

Is your feature request related to a problem? Please describe.
I am currently developing a webview using an iframe in a WASM environment. At this time, the iframe is above the egui canvas, but when dragging the tab, there is a problem with widgets and mouse events being fed to the iframe, so I would like not to show webviews while dragging. However, it's not likely to checking the dragging status in the main window.

Describe the solution you'd like
It would be nice to be able to query the dragging state in the same way as WindowState like below.

let dock_state = DockState::new(vec![]); // should be inited somewhere
let window_state = dock_state.get_main_window_state();
if window_state.dragged() {
    println!("dragging :)");
}

Describe alternatives you've considered
Otherwise, it would be a good idea to receive a UI response as below.
But it doesn't assume the tabs' dragging state.

let mut dock_state = DockState::new(vec![]); // should be inited somewhere
let response = DockArea::new(&mut dock_state).show(...);
if response.dragged() {
    println!("dragging :)");
}

Additional context
If you need any development support, I'd be happy to help!