* Moved Slate.h into SlateBasics.h and began shifting less commonly used headers into SlateExtras.h.
* Slate.h now simply includes SlateBasics.h and SlateExtras.h.
* Slate.h includes a deprecated warning now to indicate that SlateBasics.h + specific includes should be used instead.
* Moved dozens of inlined functions using Slate widgets into .cpp files to avoid header dependencies.
* All code samples now include SlateBasics.h and SlateExtras.h so future shifts will not break most those projects, but not trigger the deprecation warning of including Slate.h.
#BUN
[CL 2329610 by Wes Hunt in Main branch]
- removed dummy UClasses (no longer needed)
- removed file header comments (not used)
- removed duplicated function documentation in cpp files
- documentation cleanup, punctuation, spelling etc.
- pragma once include guards (now work on all platforms)
- relative public includes (are auto-discovered by UBT)
- fixed too many/too few line breaks
- deleted empty files
- missing override
- NULL to nullptr
[CL 2305058 by Max Preussner in Main branch]
