[FIX] web_editor: properly await all async parts for onRemove

Even though onRemove cannot be async itself (yet?), it is however still
called via a trigger_up of `call_for_each_child_snippet` so that each
part of the snippet being removed has their onRemove called. Doing that
way, some SnippetEditor instances may have to be created and it can be
an async operation... the problem is that our code was only awaiting
the first of those instanciations instead of all of them.

closes odoo#76829

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>

addons/web_editor/static/src/js/editor/snippets.editor.js

@@ -207,8 +207,8 @@ var SnippetEditor = Widget.extend({
                     for (var i in editor.styles) {
                         editor.styles[i].onRemove();
                     }
-                    resolve();
                 },
+                onSuccess: resolve,
             });
         });
 
@@ -337,7 +337,7 @@ var SnippetEditor = Widget.extend({
         }
 
         this.$target.after($clone);
-        this.trigger_up('call_for_each_child_snippet', {
+        this.trigger_up('call_for_each_child_snippet', { // FIXME should be awaited
             $snippet: $clone,
             callback: function (editor, $snippet) {
                 for (var i in editor.styles) {
@@ -1651,7 +1651,10 @@ var SnippetsMenu = Widget.extend({
      * @param {OdooEvent} ev
      */
     _onCallForEachChildSnippet: function (ev) {
-        this._callForEachChildSnippet(ev.data.$snippet, ev.data.callback);
+        const prom = this._callForEachChildSnippet(ev.data.$snippet, ev.data.callback);
+        if (ev.data.onSuccess) {
+            prom.then(() => ev.data.onSuccess());
+        }
     },
     /**
      * Called when asked to clean the DOM for save. Should technically not be