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/widgets/button-test.js.es6

@@ -11,7 +11,7 @@ widgetTest('icon only button', {
 
   test(assert) {
     assert.ok(this.$('button.btn.btn-icon.no-text').length, 'it has all the classes');
-    assert.ok(this.$('button .d-icon.d-icon-smile-o').length, 'it has the icon');
+    assert.ok(this.$('button .d-icon.d-smile-o').length, 'it has the icon');
   }
 });
 
@@ -24,7 +24,7 @@ widgetTest('icon and text button', {
 
   test(assert) {
     assert.ok(this.$('button.btn.btn-icon-text').length, 'it has all the classes');
-    assert.ok(this.$('button .d-icon.d-icon-plus').length, 'it has the icon');
+    assert.ok(this.$('button .d-icon.d-plus').length, 'it has the icon');
     assert.ok(this.$('button span.d-button-label').length, 'it has the label');
   }
 });