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/poster-name-test.js.es6

@@ -38,7 +38,7 @@ widgetTest('extra classes and glyphs', {
     assert.ok(this.$('span.staff').length);
     assert.ok(this.$('span.admin').length);
     assert.ok(this.$('span.moderator').length);
-    assert.ok(this.$('.d-icon-shield').length);
+    assert.ok(this.$('.d-shield').length);
     assert.ok(this.$('span.new-user').length);
     assert.ok(this.$('span.fish').length);
   }