[IMP] mass_mailing: add title for the templates and update thumbnail

Purpose
=======
We can now define a title for the mailing templates that will be
displayed bellow the thumbnail.

Improve the thumbnail image of the "plaintext" template.

Technical
=========
The title is set on the "title" atribute, and not on "data-title",
because the data attributes are not translated.

Task-2593231

Part-of: odoo/odoo#79174

addons/mass_mailing/static/src/js/mass_mailing_widget.js

@@ -415,6 +415,7 @@ var MassMailingFieldHtml = FieldHtml.extend({
             });
             return {
                 name: name,
+                title: $theme.attr("title") || "",
                 className: classname || "",
                 img: $theme.data("img") || "",
                 template: $theme.html().trim(),

addons/mass_mailing/static/src/xml/mass_mailing.xml

@@ -12,6 +12,7 @@
             <a t-att-id="theme.name" role="menuitem" href="#" class="dropdown-item">
                 <div class="o_thumb small"  t-attf-style="background-image: url(#{theme.img}_small.png)"/>
                 <div class="o_thumb large" t-attf-style="background-image: url(#{theme.img}_large.png)"/>
+                <h5 class="text-white text-capitalize text-truncate" t-esc="theme.title"/>
             </a>
         </t>
     </div>

addons/mass_mailing/views/snippets_themes.xml

@@ -20,17 +20,20 @@
             <t t-set="company_id" t-value="res_company"/>
             <div id="email_designer_themes">
                 <div data-name="basic"
+                     title="Plain Text"
                      data-nowrap="1"
                      data-img="/mass_mailing/static/src/img/theme_imgs/basic_thumb"
                      data-images-info='{"logo": {"format": "png"}}'>
                     <t t-call="mass_mailing.theme_basic_template"/>
                 </div>
                 <div data-name="empty"
+                     title="Start From Scratch"
                      data-img="/mass_mailing/static/src/img/theme_imgs/empty_thumb"
                      data-images-info='{"logo": {"format": "png"}}'>
                     <t t-call="mass_mailing.theme_empty_template"/>
                 </div>
                 <div data-name="default"
+                     title="Welcome Message"
                      data-img="/mass_mailing/static/src/img/theme_imgs/default_thumb"
                      data-images-info='{"logo": {"format": "png"}}'>
                     <t t-call="mass_mailing.theme_default_template"/>