Pass the DropArea to setDropIndicatorAllowedFunc() too

So the lambda can have more advanced usage and inspect the
target layout before allowing or disallowing the drop.

For our use case, we want to limit max 3 dock widgets side by side,
for example.

examples/dockwidgets/main.cpp

@@ -267,7 +267,8 @@ int main(int argc, char **argv)
         // Dock widget 8 will only be allowed to dock to the outer areasa
         auto func = [](KDDockWidgets::DropLocation location,
                        const KDDockWidgets::DockWidgetBase::List &source,
-                       const KDDockWidgets::DockWidgetBase::List &target) {
+                       const KDDockWidgets::DockWidgetBase::List &target,
+                       KDDockWidgets::DropArea *) {
             Q_UNUSED(target); // When dragging into a tab, 'target' would have the list of already tabbed dock widgets
 
             const bool isDraggingDW8 = std::find_if(source.cbegin(), source.cend(), [] (KDDockWidgets::DockWidgetBase *dw) {