Improved handling of XWayland grabs

A solution for https://github.com/Smithay/smithay/issues/1714.

With this, the lock screen is able to get keyboard focus normally, but
focus then reverts to the XWayland grab surface. This can be tested with
the example client from the issue.

Adding a new variant of `KeyboardFocusTarget` is annoying. Maybe it
could map to a different variant of the enum. But it presumably needs to
handle any `wl_surface` XWayland uses. (Override redirect surfaces?
Subsurfaces?) This seems as good as anything for now.

src/debug.rs

@@ -344,6 +344,7 @@ fn format_keyboard_focus(focus: Option<KeyboardFocusTarget>) -> String {
         Some(Popup(x)) => format!("Popup {}", x.wl_surface().id().protocol_id()),
         Some(Group(_)) => format!("Window Group"),
         Some(LockSurface(x)) => format!("LockSurface {}", x.wl_surface().id().protocol_id()),
+        Some(XWaylandGrab(x)) => format!("XWayland Grab {}", x.id().protocol_id()),
         None => format!("None"),
     }
 }