Make semantics not editable from metadata editor (#3924)

Semantics metadata is (correctly) excluded from the Metadata section
menu on the Items page.

However, you can still use 'Add Metadata' and manually type 'semantics'
as custom namespace. If there was no semantics metadata, that enables
you to add semantics metadata to the managed provider.

This makes the semantics namespace non-editable in the editor.

Signed-off-by: Mark Herwege <mark.herwege@telenet.be>

Author: mherwege

bundles/org.openhab.ui/web/src/pages/settings/items/metadata/item-metadata-edit.vue

@@ -165,6 +165,7 @@ export default {
       }
     },
     editable () {
+      if (this.namespace === 'semantics') return false
       return this.metadata.editable !== false
     }
   },