Add uniqueId() and removeUniqueId()

[k_borchers]

Generalize the automatic generation of id's for elements when necessary. We automatically generate id's for a number of widgets including menu/menu items, dialog title bars, etc. This will provide a standard way for generating those id's as well as removing them when the widget is destroyed.

[Kris Borchers]

Core: Add the uniqueId() and removeUniqueId() methods written by @scottgonzalez to provide a generalized way of generating and removing generated element id's. Also, added a unit test. Fixed #8361 - Add uniqueId() and removeUniqueId()

Changeset: 40e47c0b08c31e5003833b9c46d36e7468a63de8