Add DropLocation_Horizontal and DropLocation_Vertical

So we can quickly use a bitwise and to see if an arbitrary
drop location is vertical or horizontal.

src/private/DropIndicatorOverlayInterface.cpp

@@ -112,6 +112,8 @@ KDDockWidgets::Location DropIndicatorOverlayInterface::multisplitterLocationFor(
         return KDDockWidgets::Location_None;
     case DropLocation_Inner:
     case DropLocation_Outter:
+    case DropLocation_Horizontal:
+    case DropLocation_Vertical:
         qWarning() << Q_FUNC_INFO << "Unexpected drop location" << dropLoc;
         break;
     }