SDK-18837 PopUpMenuButton's iconField property is not exposed
Added iconField and iconFunction properties to PopUpMenuButton. Also, fixed bug where the anchor wasn't instantiating the icon of the selectedItem when it is first displayed. The problem was that we were creating the Menu popUp in the dataProvider setter. At that time, the PUMB hasn't been attached to the stage. This causes the call in getPopUp to not fully add the menu popUp to the PopUpManager via addPopUp. When the PUMB attempts to access the icon by calling itemToIcon, it fails because it doesn't have a reference to a SystemManager.
The fix is to move the creation of the popUp menu to commitProperties. At that point, the PUMB will be attached to the displayList and the call to PopUpManager.addPopUp will fully complete.
QE Notes: Add tests for iconField and iconFunction properties
Doc Notes: Add documentation for iconField and iconFunction properties