Provides a method to get xdg_toplevel on Wayland

[ultimaweapon]

Description

I would like to create xdg_dialog_v1 object for winit window. In order to create it I need xdg_toplevel from winit window to feed to xdg_wm_dialog_v1::get_xdg_dialog.

Relevant platforms

Wayland

[ultimaweapon]

I propose adding a new method to WindowExtWayland like the following:

pub trait WindowExtWayland {
    fn xdg_toplevel(&self) -> Option<&XdgToplevel>;
}

I can create a PR to implement this if the above proposal is okay.