When adding a new layer, insert it above the current one

It is usually more useful for the layer to be inserted above the current
layer, rather than always at the top of the stack.

src/tiled/mapdocument.cpp

@@ -470,7 +470,8 @@ Layer *MapDocument::addLayer(Layer::TypeFlag layerType)
     }
     Q_ASSERT(layer);
 
-    const int index = mMap->layerCount();
+    const int index = mCurrentLayerIndex >= 0 ? mCurrentLayerIndex + 1
+                                              : mMap->layerCount();
     mUndoStack->push(new AddLayer(this, index, layer));
     setCurrentLayerIndex(index);