UETOOL-213 - Minimize Slate FString -> FText conversion (remove SLATE_TEXT_ATTRIBUTE)
This fixes any editor/engine specific code that was passing text to Slate as FString rather than FText.
[CL 2399803 by Jamie Dale 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]
#UE4 Added support to drag and drop world assets on the level browser (specifically the level list in SWorldHierarchy) to add them as sublevels.
#codereview Dmitriy.Dyomin
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Integrated using branch Ue4-To-UE4-Fortnite-Simple (reversed) of change#2222841 by Bob.Tellez on 2014/07/17 18:26:40.
[CL 2227532 by Bob Tellez in Main branch]
Refactored some of the UI code for ULevelStreaming to use PostEditChangeProperty so it's more reuseable.
#codereview matt.kuhlenschmidt,dmitriy.dyomin
[CL 2222165 by Chris Gagnon in Main branch]
Splitted World Browser views into three independent windows, each window can be summoned from a hierarchy view or details view toolbar
World composition can now be toggled through WorldSettings - bEnableWorldComposition. Once enabled persistent level will be treated as a world root. No more '?worldcompistion' parameter workarounds. 'Open World' command was removed, as levels saved with world composition enabled can be open through 'Open Level' command.
Added bEnableWorldOriginRebasing parameter to WorldSettings, which contols whether world origin shifting should be used
[CL 2072584 by Dmitriy Dyomin in Main branch]
