NSM: slightly optimized SaveModelState().
Interfaces: added IDNDTab.
Core: handle DND tabs.
Poshuku: support DND for tabs.
Interfaces: use QDragMoveEvent instead of QDragEnterEvent.
Core: handle drag moves.
Azoth: ChatTab implements IDNDTab.
Monocle: support DND; DocumentTab now implements IDNDTab.