feat(pat-sortable): Add the sortable-item class to each sortable element.

The current situation requires the integrator to add the `sortable-item` class
on all sortables manually. If that is not done there can be styling problems
like a missing insert marker which makes it hard to use the pattern. As the
pattern already defines what should be a sortable it we also let the pattern
assign the class "sortable-item".

Author: thet

src/pat/sortable/sortable.js

@@ -41,6 +41,8 @@ export default Base.extend({
         this.$sortables = this.$el.children().filter(this.options.selector);
         this.$sortables.each(function (idx) {
             $(this).data("patterns.sortable", { position: idx });
+            // Add `.sortable-item` class to each sortable.
+            this.classList.add("sortable-item");
         });
         return this;
     },