[TIMOB-6270] iOS: TextField leftButton/rightButton properties should have better type checking

As documented, the TextField leftButton and rightButton properties will take any type of object. But it appears that the only object that can be usefully applied here is a Button. iOS platform was reluctant to document the type as Ti.UI.Button in the TDoc since at this point any object can be inserted here, although it may not do anything useful. (It appears that you can use an ActiviyIndicator as well.) It would be good to be able to provide a useful type value for this property so developers know what type or types can specify here. And we should probably throw an error if they pass in something else.