display: Fix inconsistent behavior with demand attention

When focus stealing prevention kicks in, mutter would set the demand
attention flag on the window.

Focus stealing prevention would also prevent the window from being
raised and focused, which is expected as its precisely its purpose.

Yet, when that occurs, the user expects the window which has just been
prevented from being focused to be the next one in the MRU list, so
that pressing [Alt]-[Tab] would raise and give focus to that window.

This works fine when the window is placed on the primary monitor, but
not when placed on another monitor, in which case the window which has
been denied focus is placed ahead of the MRU list and pressing
[Alt]-[Tab] would leave the focus on the current window.

This is because of a mechanism in `meta_display_get_tab_list()` which
forces the windows with the demand attention flag set to be placed first
in the MRU list when they're placed on a workspace different from the
current one.

But because workspaces apply only to the primary monitor (by default),
the windows placed on other outputs have their workspace set to `NULL`
which forces them ahead of the MRU list by mistake.

Fix this by using the appropriate `meta_window_located_on_workspace()
function to check if the window is on another workspace.

Closes: https://gitlab.gnome.org/GNOME/mutter/issues/523

src/core/display.c

@@ -2236,7 +2236,7 @@ meta_display_get_tab_list (MetaDisplay   *display,
         MetaWindow *l_window = w->data;
 
         if (l_window->wm_state_demands_attention &&
-            l_window->workspace != workspace &&
+            !meta_window_located_on_workspace (l_window, workspace) &&
             IN_TAB_CHAIN (l_window, type))
           tab_list = g_list_prepend (tab_list, l_window);
       }