cleanup
Registry.Pooled<T> introduced
ISplitView.ShowInNewWindow introduced
bugs: ThingGraphExtensions Marker check if null | GraphSceneExtensions: delete item if its a single Edge | GraphSceneFolding: use Key.Plus & Shift as Asterisk |GraphSceneDisplay: improve clear |  VisualsDragDropAction: hover Focused (make a link if sink is Focused) ) | GraphVisualEntityMapping doesnt chain
bug: dont show Sheet in CurrentDisplay
bug: Swf.Converter: corrrect convert Keys  Add, NumPadAdd, NumPadSubtract, Minus, Plus, Asterisk
bug: StreamContentUiManager saves even if content.Source == null
Merge: allow only merge of strings and thinggraph-based graphs
GraphSceneMesh: add GraphSceneMeshExtensions ClearDisplaysOf, DisplaysOfBackGraph, ScenesOfBackGraph
SplitView: ChangeData uses Mesh => supports more than 2 displays
SheetViewer: register on GraphSceneMesh