[REF] qweb: improve t-on generated code

and deduplicate logic between directive t-on applied on a node and on a
component

src/qweb/compilation_context.ts

@@ -46,12 +46,12 @@ export class CompilationContext {
    * Such a key is necessary when we need to associate an id to some element
    * generated by a template (for example, a component)
    */
-  generateTemplateKey(): string {
+  generateTemplateKey(prefix: string = ""): string {
     const id = this.generateID();
     if (this.loopNumber === 0 && !this.currentKey) {
-      return `'__${id}__'`;
+      return `'${prefix}__${id}__'`;
     }
-    let locationExpr = `\`__${id}__`;
+    let locationExpr = `\`${prefix}__${id}__`;
     for (let i = 0; i < this.loopNumber - 1; i++) {
       locationExpr += `\${i${i + 1}}__`;
     }