MAGN-3210 : User gets stuck in selection mode when selection box is dragged out of canvas.

This fix is for clearing the selection mode when the selection box is dragged out of canvas.

When the nodes are selected using windows selecting and if the mouse pointer is dragged outside the canvas, the selection box will be cleared automatically.

http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-3210 

- [ ]  @Benglin 
Please use ```IsMouseCaptured``` property, not ```CaptureMouse``` method.