Bug Tool-tip remains visible(switch to another program) fixed.
"new CustomPopupPlacementCallback" and "new EventHandler" removed.
Instead of Application.Current.MainWindow used DynamoView.
Merge branch 'Sitrus2' into DUI-Bug_TooltipVisible(SwitchToAnotherProgram)
Comment updated.
Merge pull request #2254 from vmoyseenko/DUI-Bug_TooltipVisible(SwitchToAnotherProgram)

DUI: Bug Tool-tip remains visible(switch to another program)