[FIX] website_animate: fix save animated snippet

Before this commit, when dropping custom snippet (or saved snippet)
containing an animation in the page, the animated element of the
snippet remained hidden.

task-2664876

closes odoo/design-themes#517

X-original-commit: 17d4df53f9
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>

website_animate/static/src/js/o_animate.editor.js

@@ -22,6 +22,12 @@ function forceAnimation() {
 
 //  Animations
 sOptions.registry.o_animate = sOptions.Class.extend({
+    /**
+     * @override
+     */
+    async onBuilt() {
+        this.$target[0].classList.toggle('o_animate_preview', this.$target[0].classList.contains('o_animate'));
+    },
 
     //--------------------------------------------------------------------------
     // Options