popover: raise when showing

Some other widget might have mapped and raised another child window of
the toplevel in the meantime, causing the popover window to be covered.

Raise the popover window to avoid the issue.

Bugzilla: https://bugzilla.gnome.org/show_bug.cgi?id=763627

gtk/gtkwindow.c

@@ -7623,7 +7623,7 @@ gtk_window_restack_popovers (GtkWindow *window)
       GtkWindowPopover *popover = link->data;
       link = link->next;
 
-      if (popover->window && gdk_window_is_visible (popover->window))
+      if (popover->window)
         gdk_window_raise (popover->window);
     }
 }
@@ -12398,6 +12398,7 @@ _gtk_window_raise_popover (GtkWindow *window,
       g_list_free (link);
       break;
     }
+  gtk_window_restack_popovers (window);
 }
 
 static GtkWidget *inspector_window = NULL;