[FIX] website_slides: add id and hint to oe_structures

In these two cases, adding and id (and editor message when it is visible)
enables
1/ to retain customizations when the view is updated
2/ to better signal that the section on slides is not meant to be
customizable per-record.

In particular, it was not clear that adding a block above a slide content would
make it visible on all slides. We therefore add this hint there, and not in
other places where that scope is obvious or where it isn't shown even if added,
or where it causes crashes (above covers).

Task-3120276
opw-3106887

closes odoo/odoo#111445

X-original-commit: 24977381fc54e258fa05724a9aa69c72656ece9c
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>

addons/website_slides/views/website_slides_templates_homepage.xml

@@ -324,7 +324,7 @@
 
 <template id='courses_footer'>
     <section class="s_banner">
-        <div class="oe_structure oe_empty"/>
+        <div class="oe_structure oe_empty" id="oe_structure_website_slides_course_footer_1"/>
     </section>
 </template>
 

addons/website_slides/views/website_slides_templates_lesson.xml

@@ -294,7 +294,8 @@
             <a t-att-href="'/slides/%s/tag/%s' % (slug(slide.channel_id), slug(tag))" class="badge text-bg-info py-1 px-2" t-esc="tag.name"/>
         </t>
     </div>
-    <div class="oe_structure oe_empty"/>
+    <div class="oe_structure oe_empty" id="oe_structure_website_slides_lesson_top_1"
+         data-editor-message="BUILDING BLOCKS DROPPED HERE WILL BE SHOWN ACROSS ALL LESSONS"/>
     <div t-if="slide.slide_category == 'infographic'" class="o_wslides_lesson_content_type" t-field='slide.image_1920' t-options="{'widget': 'image', 'style': 'width: 100%;'}"/>
     <div t-else="" class="o_wslides_lesson_content_type">
         <div t-if="slide.slide_category == 'document'" class="ratio ratio-4x3 embed-responsive-item mb8" style="height: 600px;">