window-slot: Update back forward actions upon restore

After commit [1], back and forward actions are not correctly reenabled after restoring a tab. The actions were previously reenabled after restoring a tab due to the fact that the restored tab was not automatically active. Once the tab was created and the user switched to the newly restored tab that would fire a location change which in turn called nautilus_window_sync_location_widgets() where the back and forward actions would get enabled. Now that the actions live in the slot, this no longer happens. Reenable the actions after the back and forward lists have been restored.

[1] 2d6f4b82