Generic tool for moving a component’s position relative to its container
Called when the drag operation starts.
The event parameter is the mouse event that established the drag operation; its x and y attributes correspond to the current location of the mouse, and not to the position of the mouse when the initial left_down or right_down event happened.
This method is called for every mouse_move event that the tool receives while the user is dragging the mouse.
It is recommended that subclasses do most of their work in this method.
Returns whether the (x,y) position is in a region that is OK to drag.
Used by the tool to determine when to start a drag.