UE-14641 - Fix PushMenu() to use QueryPopupMethod()
Pretty big refactor
Adds IMenu as way to identify menus. Replaces referring to menus as SWindows.
Lots of uses of PushMenu() fixed up to match new API
#codereview Nick.Atamas
[CL 2579277 by Chris Wood 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]
