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]
#ttp 340965 - UE4-Fortnite: Editor: levels: every time you toggle visibility on a level it makes all Layers visible, yikes
#branch UE4
#change additionally, making a level current no longer forces the layer visibility either.
reviewed by Thomas.Sarkanen
[CL 2217507 by Andrew Brown 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]
- Added icons to level hierarchy to distinguish level streaming methods
- Added tile coordinates offset when umporting tiled heightmap
[CL 2047641 by Dmitriy Dyomin in Main branch]
