shell: Put xwayland surface at the top of the fullscreen layer

An xwayland surface corresponds to a override-redirect window under X,
which is typically a menu or a popup window.  They typically appear
with a keyboard and mouse grab and by nature of being override-redirect
these window can appear anywhere on screen and in the stack.

We need to resort to heuristics to decide where to place the
override-redirect in our surface stack, and for now we'll just put it on
top of everything.  That's going to be correct for almost all cases of
clicking to open a menu, but we can revisit and refine if we run into
a case that needs better handling.

desktop-shell/shell.c

@@ -2011,6 +2011,8 @@ shell_surface_calculate_layer_link (struct shell_surface *shsurf)
 	}
 
 	case SHELL_SURFACE_XWAYLAND:
+		return &shsurf->shell->fullscreen_layer.view_list;
+
 	case SHELL_SURFACE_NONE:
 	default:
 		/* Go to the fallback, below. */