Web UI - Leverage validation report client side and allow async layout validation

// Trigger the layout ASYNC validation if it exists.
$validate() {
    return this.$.myOp.execute()
        .then((ret) => {
            if (!ret || !ret.value) {
                 const element = this._getValidatableElement();
                 element.invalid = true;
                 element.errorMessage = "we have a pb here";
                 return Promise.reject('Pre validation service problem.');
            }
        });
},

_create() {
    const elementsToValidate = this._getValidatableElements();
    let valid = true; 
    let validations = [];
    elementsToValidate.forEach(el => {
        if (valid && el.validate) {
            valid = el.validate();
        }
        if (valid && el.$validate) {
            validations.push(el.$validate());
        }
    });
    if (!valid) {
        return;
    }

    Promise.all(validations)
        .then(() => {
            this.document.name = this.document.name || this._sanitizeName(this.document.properties['dc:title']);
            this.set('creating', true);
            return this.$.docRequest.post();
        })
        .then((response) => {
            this.$.creationStats.storeType(this.selectedDocType.id);
            this._clear();
            this.navigateTo('browse', response.path);
            this._notify(response);
            this.set('creating', false);
        })
        .catch((err) => {
            this.set('creating', false);
            this.fire('notify', {message: this.i18n('documentCreationForm.createError')});
            console.error(err);
        });
},