[Drag] Add support for key interactions in drag mode

[fipro78]

When an IDragMode is active, there is currently no further interaction possible despite the mouse related ones. That means, pressing a key while a drag mode is active has no effect.

There are use cases where the drag behavior should change in case a key is pressed. One example could be a copy or move behavior dependent on whether the CTRL key is pressed while dragging or not. This is a behavior you can see for example in the Windows Explorer.

In most of the cases this behavior is not necessary. Therefore a new IDragModeWithKeySupport interface should be introduced, that allows to define the handling of key interactions in the active drag mode. In the DragModeEventHandler the key events should be only handled and forwarded to the drag mode if the drag mode is of type IDragModeWithKeySupport.