[FIX] web: can focus out then focus in when currently being selected

Before this fix, in studio when the user select the field then focus out
then focus in and selected a child field, an exception is triggered because
the last page is visible but the data are removed.
A test in enterprise (web_studio) is added with this commit.

addons/web/static/src/js/widgets/model_field_selector.js

@@ -270,7 +270,6 @@ var ModelFieldSelector = Widget.extend({
 
         if (this.dirty) {
             this.dirty = false;
-            this.pages = this.pages.slice(0, this.chain.length || 1);
             this.trigger_up("field_chain_changed", {chain: this.chain});
         }
     },