Constgen V2.

Design Goal:
1. Keep sync with wxWidgets code:
    a) Get things right. EventType for example, must be vars in go.
    b) Survive  enum value altering in wxWidgets code.
    c) Deletion in wxWidgets causes failure of build.
2. Try best to make them go consts.
    a) All consts in <wx/defs.h>(C header) are consts in go.
    b) Enums in C++ headers are vars in go.