Change GUI Object Setting values to be members of the IGUIObject inheriting class, rename AddSetting to RegisterSetting.

Improves performance for Draw calls by 3-5% according to a shady benchmark.
Improves memory layout, since the values are not on the heap anymore but in the using class.
Reduces complexity of the implementation and increases type safety.
Allows specifying default values at setting value construction time, refs D2242.
Inspired by Vladislav introducing members that cached GetSetting values in rP19479/D325, refs #4039, rP20075/D763, refs 4225, rP21429/D474, D406, which were formerly proposed to be removed in D2241.

Differential Revision: https://code.wildfiregames.com/D2313
Tested on: clang 8.0.1, gcc 9.1.0, Jenkins vs2015
Comments By: Vladislav