Ajax quick plot enhancements: mouse cursor and event controlling

[wuma]

1. It's an issue: when move the mouse pointer to the center point of the capture box, the cursor is "rotate". The correct cursor should be "move". I fixed it by moving the "start point" of the rotate handler to the "capture box" canvas.

2. It's an enhancement: add a method to suppress all events for the capture box, like move, rotate, warning message etc. Then some functions could use quick plot to draw only a rectangle on screen. In these cases, the events are not necessary.

Because the fix and the enhancement are connected closely, I just combine them into a single submission.