#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]
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]