CHanged DockAreaTabBar to handle the dragging state via DragState member variable instead of testing for FloatingWidget nullptr

src/DockWidgetTab.cpp

@@ -95,7 +95,7 @@ struct DockWidgetTabPrivate
 	/**
 	 * Test function for current drag state
 	 */
-	bool isDraggingState(eDragState dragState)
+	bool isDraggingState(eDragState dragState) const
 	{
 		return this->DragState == dragState;
 	}