Adds debug-mode only property validation to the layout property. Argument and property validation helps developers save time by failing at the root of the problem, providing useful messages and avoiding difficult to diagnose infinite loops. Normally, this type of validation would come at the expense of the performance of an application, but fortunately SproutCore's build tools allow us to separate all of this code entirely from production builds. Win-win!
Fixes an extremely rare security vulnerability in SC.TextFieldView that could occur if a hint overlay is used that included user generated hint. Also fixes @type of applyImmediately.