Move CGUI constructor to the top of its file following rP9.

Move object type definitions to own file, since CGUI doesn't use the GUIObject types otherwise and for similarity with GUISettingTypes, making it slightly easier to maintain and read.

Differential Revision: https://code.wildfiregames.com/D2439
Tested on: clang 9.0.0, Jenkins/vs2015, Jenkins/gcc6