Added the ability for asset type actions to provide an extra overlay layer for their asset thumbnails
UE-7970 - Add small 'play' button to EditorTutorial asset thumbnail
Also cleaned up FAssetThumbnail::MakeThumbnailWidget so that it took a config struct (FAssetThumbnailConfig) rather than 7 or 8 defaulted parameters.
[CL 2419655 by Ben Marsh 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]