[IMP] web_editor: add a new string attribute on snippet option

This commit introduces a new way to set the title of an option section
in the customize panel. The title can now be filled in a new "string"
attribute in the template of an option.

See merge commit for more information.

task-2710582

addons/web_editor/static/src/js/editor/snippets.editor.js

@@ -759,6 +759,9 @@ var SnippetEditor = Widget.extend({
             if (!val.selector.is(this.$target)) {
                 return;
             }
+            if (val.data.string) {
+                $optionsSection[0].querySelector('we-title > span').textContent = val.data.string;
+            }
             if (val['drop-near']) {
                 this.selectorSiblings.push(val['drop-near']);
             }

addons/web_editor/views/snippets.xml

@@ -222,7 +222,7 @@
             />
         </div>
 
-        <div data-js="BackgroundShape" string="Shape"
+        <div data-js="BackgroundShape"
              t-att-data-selector="selector"
              t-att-data-exclude="exclude"
              t-att-data-target="target">