Keep xwayland stacking order in sync when switching workspaces

Until we expose the workspaces to xwayland we need a way to
ensure that xwayland views on the current workspace are always
stacked above xwayland views on other workspaces.

If we fail to do so, issues arise in scenarios where we change
the mouse focus but do not change the (xwayland) stacking order.

Reproducer:
- If followMouse is enabled, raiseOnFocus must be disabled
- Open at least two xwayland windows which allow scrolling
  (some X11 terminal with 'man man' for example)
- Switch to another workspace, open another xwayland window
  which allows scrolling and maximize it
- Switch back to the previous workspace with the two windows
- Move the mouse to the xwayland window that does *not* have
  focus
- Start scrolling
- All scroll events should end up on the maximized window on
  the other workspace

This patch fixes the issue by simply raising all windows from
the current workspace again in their original stacking order
when switching workspaces.

Reported-by: Domo via IRC (thanks!)

include/xwayland.h

@@ -46,6 +46,8 @@ void xwayland_unmanaged_create(struct server *server,
 void xwayland_view_create(struct server *server,
 	struct wlr_xwayland_surface *xsurface, bool mapped);
 
+void xwayland_adjust_stacking_order(struct server *server);
+
 struct wlr_xwayland_surface *xwayland_surface_from_view(struct view *view);
 
 void xwayland_server_init(struct server *server,