[FIX] web_editor: Fieldtexthtml wait until iframe loaded to save

In the case where:
- you are too fast editing a mass mailing, trigger an onchange (e.g. mailing_model) and then save
You can use the edit/save shortcuts to make yourself believe you are Flash Gordon
- your internet is too slow (Down: 4Mb/s, Up: 500kb/s, Latency: 20ms)

Before ths commit, the onchange will return *before* the field (the iframe actually) is loaded.
This onchange contains a signal *in* the body_html of the field to allow communication with the iframe.
If you save before the iframe is loaded, this body_html keyword will be saved.

After this commit we deactivate the save button until the iframe is loaded.
We could have gone for a python user error on the value of the body_html too.
But it would have added a supplementary click and potential reloading

OPW 783616

addons/web_editor/static/src/js/backend.js

@@ -293,6 +293,8 @@ var FieldTextHtml = widget.extend({
         var self = this;
         this.$el.closest('.modal-body').css('max-height', 'none');
         this.$iframe = this.$el.find('iframe');
+        // deactivate any button to avoid saving a not ready iframe
+        $('.o_cp_buttons, .o_statusbar_buttons').find('button').addClass('o_disabled').attr('disabled', true);
         this.document = null;
         this.$body = $();
         this.$content = $();
@@ -318,6 +320,8 @@ var FieldTextHtml = widget.extend({
         this.lang = this.lang ? this.lang[1] : this.view.dataset.context.lang;
         this._dirty_flag = false;
         this.render_value();
+        // reactivate all the buttons when the field's content (the iframe) is loaded
+        $('.o_cp_buttons, .o_statusbar_buttons').find('button').removeClass('o_disabled').attr('disabled', false);
         setTimeout(function () {
             self.add_button();
             setTimeout(self.resize,0);