Insert Highlighting is not always an appropriate dragging behaviour

It is not appropriate to allow the specific insert point to be configured for some drag operations. For example, when dragging a folder around, the order of the folder within its parent is irrelevant. In this case the dragging mode should be simply to highlight the parent node that the item is added to as per Explorer.

Added the DragStyle property, which allows the style to be configured to either etvwInsertMark or etvwDropHighlight. The default is Insert Mark for compatibility with the first release of the control.