lint if tileset properties are set on the map instead

(this seems to be a common mistake so far)

lib/Properties.hs

@@ -116,6 +116,10 @@ checkMapProperty p@(Property name _) = case name of
       unwrapString p $ \str ->
         unless ("https://static.rc3.world/scripts" `isPrefixOf` str)
         $ forbid "only scripts hosted on static.rc3.world are allowed."
+    | name `elem` ["jitsiRoom", "bbbRoom", "playAudio", "openWebsite"
+                  , "url", "exitUrl", "silent", "getBadge"]
+          -> complain $ "property " <> name
+                      <> " should be set on layers, not the map directly"
     | otherwise
           -> complain $ "unknown map property " <> prettyprint name
 
@@ -342,7 +346,7 @@ checkTileLayerProperty p@(Property name _value) = case name of
           if | isOldStyle path ->
                complain "Old-Style inter-repository links (using {<placeholder>}) \
                         \cannot be used at rC3 2021; please use world:// instead \
-                        \(cf. howto.rc3.world)."
+                        \(see howto.rc3.world)."
              | ext == "tmx" ->
                  complain "Cannot use .tmx map format; use Tiled's json export instead."
              | ext /= "json" ->