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()
```

config/locales/client.el.yml

@@ -1040,7 +1040,7 @@ el:
       shift: ' Shift'
       ctrl: 'Ctrl'
       alt: 'Alt'
-    select_box:
+    select_kit:
       default_header_text: Επίλεξε...
       no_content: Δεν βρέθηκαν αποτελέσματα
       filter_placeholder: Αναζήτηση...