Reintroduce ability to query use of X11 or Wayland as backend on raw EventLoop

[vorporeal]

Description

Pre 0.30, EventLoopWindowTargetExtX11 enabled users to invoke is_x11 on an EventLoop, as an EventLoop could be coerced into an EventLoopWindowTarget.

With the 0.30 API changes that replaced EventLoopWindowTarget with ActiveEventLoop, there is no longer a way to perform this check without doing something indirect/awkward like doing a match on event_loop.owned_display_handle().raw_display_handle().

This functionality was used in our application to determine whether to invoke winit::platform::x11::register_xlib_error_hook before starting the event loop. While we could set up our error hook callback as part of the init event, we'd prefer to do it before starting the event loop for maximum "coverage".

Relevant platforms

X11

[kchibisov]

Yeah, probably was just forgotten. Using raw-window-handle and matches! should be fine as well.

Though, in the future such identity may go away anyway when the backends could be plugged/etc.