add lint for empty group layers

lib/Properties.hs

@@ -118,9 +118,16 @@ checkLayer = do
 
   case layerType layer of
     "tilelayer" -> mapM_ checkLayerProperty (getProperties layer)
+    "group" -> pure ()
     ty -> unless (layerName layer == "floorLayer" && ty == "objectgroup")
           $ complain "only tilelayer are supported."
 
+  if layerType layer == "group"
+    then when (null (layerLayers layer))
+    $ warn "Empty group layers are pointless."
+    else when (isJust (layerLayers layer))
+    $ complain "Layer is not of type \"group\", but has sublayers."
+
 
 -- | Checks a single (custom) property of a layer
 --