Initial submenu support.

package.json

@@ -17,6 +17,7 @@
   "dependencies": {
     "@cycjimmy/jsmpeg-player": "^5.0.1",
     "@material/image-list": "^12.0.0",
+    "@material/rtl": "^13.0.0",
     "custom-card-helpers": "^1.8.0",
     "dayjs": "^1.10.7",
     "dlv": "github:developit/dlv",