[FIX] qweb: properly remove t-ref reference

when the node is removed from the dom

src/qweb/extensions.ts

@@ -93,6 +93,7 @@ QWeb.addDirective({
     const refKey = `ref${ctx.generateID()}`;
     ctx.addLine(`const ${refKey} = ${ctx.interpolate(value)};`);
     addNodeHook("create", `context.__owl__.refs[${refKey}] = n.elm;`);
+    addNodeHook("destroy", `delete context.__owl__.refs[${refKey}];`);
   }
 });
 

src/qweb/qweb.ts

@@ -69,7 +69,8 @@ const whitespaceRE = /\s+/g;
 const NODE_HOOKS_PARAMS = {
   create: "(_, n)",
   insert: "vn",
-  remove: "(vn, rm)"
+  remove: "(vn, rm)",
+  destroy: "()"
 };
 
 interface Utils {