UE-9790 - Fixing widget reflector to not crash on UMG widgets that have no associated blueprint asset.
#lockdown Matt.Kuhlenschmidt
[CL 2449857 by Matthew Griffin in Main branch]
* Removed SWidget:: GetCreatedInFile, GetCreatedInFileFName, GetCreatedInLineNumber.
* Added GetCreatedInLocation() -> FName as a non-redundant replacement, updated all code usages.
* This shrinks SWidget size by 16 bytes and reduces ambiguity on which ones to use.
* Implemented ToString() in terms of GetReadableLocation().
#codereview:max.preussner
[CL 2430545 by Wes Hunt in Main branch]
* 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]
