MagTrace tool:  implemented functionality to resume the tracing of the existing Spline
a) Select the desired spline
b) Press MagTrace button.  Operation will resume from the last point.

Magtrace tool:  prevented the "Double entry" error when user presses snap button before the previous snap operation is finished.

MagTrace tool:  fixed crash when the user clicks within the document window, but not on the image.

PolyTool\MagTrace tool:  don't perform any action if the mouse cursor is out of client are.
(affected:  MouseMove and MouseLButtonDown)