Introduces select-kit

* renames `select-box-kit` into `select-kit`
* introduces `single-select` and `multi-select` as base components
* introduces {{search-advanced-category-chooser}} as a better component for selecting category in advanced search
* improves events handling in select-kit
* recreates color selection inputs using {{multi-select}} and a custom {{selected-color}} component
* replaces category-selector by a component using select-kit and based on multi-select
* improves positioning of wrapper
* removes the need for offscreen, and instead use `select-kit-header` as a base focus point for all select-kit based components
* introduces a formal plugin api for select-kit based components
* introduces a formal pattern for loading and updating select-kit based components:

```
computeValue()
computeContent()
mutateValue()
```

test/javascripts/components/categories-admin-dropdown-test.js.es6

@@ -5,15 +5,15 @@ componentTest('default', {
   template: '{{categories-admin-dropdown}}',
 
   test(assert) {
-    const $selectBox = selectBox('.categories-admin-dropdown');
+    const $selectKit = selectKit('.categories-admin-dropdown');
 
-    assert.equal($selectBox.el.find(".d-icon-bars").length, 1);
-    assert.equal($selectBox.el.find(".d-icon-caret-down").length, 1);
+    assert.equal($selectKit.el.find(".d-bars").length, 1);
+    assert.equal($selectKit.el.find(".d-caret-down").length, 1);
 
-    expandSelectBoxKit();
+    expandSelectKit();
 
     andThen(() => {
-      assert.equal($selectBox.rowByValue("create").name(), "New Category");
+      assert.equal($selectKit.rowByValue("create").name(), "New Category");
     });
   }
 });