ISIS-537 Use opacity 0.5 for the veil. This is the same opacity value as Modal's backdrop so there is no flickering when a Modal is opened and some Ajax call is made
ISIS-537 Do not re-add the ScalarPanel's AjaxFormComponentUpdatingBehavior again and again even when rebuilding the whole GUI for the panel

https://trello.com/c/DgiUR5Cj/97-improve-the-behavior-of-the-select2-components