Providing keyboard modifiers for drag and drop operations

NOTE This is not the TModifierKey class itself, it's an abstract class that can allow future dynamic embellishment of the interaction interface.

The scrap interaction modifier is given to the drag source when the drag source is asked to provide all of the scrap items for the document being dragged.

Only the drag source sees the modifier value--the modifier is a logical modification of the sourcing of the dragged item. If the interpretation of the modification has to be deferred until the drop point, then the drag source has to package the relevant modifier state into the scrap item that it publishes for the acceptor.