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.

src/private/DropIndicatorOverlayInterface.cpp

@@ -161,7 +161,8 @@ bool DropIndicatorOverlayInterface::dropIndicatorVisible(DropLocation dropLoc) c
     }
 
     if (auto dropIndicatorAllowedFunc = Config::self().dropIndicatorAllowedFunc()) {
-        if (!dropIndicatorAllowedFunc(dropLoc, source, target))
+        DropArea *dropArea = DragController::instance()->dropAreaUnderCursor();
+        if (!dropIndicatorAllowedFunc(dropLoc, source, target, dropArea))
             return false;
     }