Keep closed custom node workspaces in background

[REACH-128 Redesign WorkspaceResolver](http://adsk-oss.myjetbrains.com/youtrack/issue/REACH-128):
"If we open a custom node from workspaces list this cn will be added
into nodes list to have a possibility to create its instances but if we
close this tab this cn won't be removed from the list and we'll get an
error on trying to create this cn instance;"

My solution is to keep custom node workspaces in background rather than delete. So all custom nodes in search list are available to be created.
The same behavior has Dynamo - on closing a custom node tab its workspace isn't deleted.
it should select another workspace to make it current among not background workspaces