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()
```

app/assets/javascripts/select-kit/components/notifications-button/notifications-button-row.js.es6

@@ -0,0 +1,37 @@
+import DropdownSelectBoxRoxComponent from "select-kit/components/dropdown-select-box/dropdown-select-box-row";
+import { buttonDetails } from "discourse/lib/notification-levels";
+import computed from "ember-addons/ember-computed-decorators";
+import { iconHTML } from 'discourse-common/lib/icon-library';
+
+export default DropdownSelectBoxRoxComponent.extend({
+  classNames: "notifications-button-row",
+
+  i18nPrefix: Ember.computed.alias("options.i18nPrefix"),
+  i18nPostfix: Ember.computed.alias("options.i18nPostfix"),
+
+  @computed("computedContent.value", "i18nPrefix")
+  title(value, prefix) {
+    const key = buttonDetails(value).key;
+    return I18n.t(`${prefix}.${key}.title`);
+  },
+
+  @computed("computedContent.name", "computedContent.originalContent.icon")
+  icon(contentName, icon) {
+    return iconHTML(icon, { class: contentName.dasherize() });
+  },
+
+  @computed("_start")
+  description(_start) {
+    return Handlebars.escapeExpression(I18n.t(`${_start}.description`));
+  },
+
+  @computed("_start")
+  name(_start) {
+    return Handlebars.escapeExpression(I18n.t(`${_start}.title`));
+  },
+
+  @computed("i18nPrefix", "i18nPostfix", "computedContent.name")
+  _start(prefix, postfix, contentName) {
+    return `${prefix}.${contentName}${postfix}`;
+  },
+});