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/topic-notifications-button-test.js.es6

@@ -1,34 +1,34 @@
 import componentTest from 'helpers/component-test';
 import Topic from 'discourse/models/topic';
 
-const buildTopic = function() {
+const buildTopic = function(level) {
   return Topic.create({
     id: 4563,
     title: "Qunit Test Topic",
     details: {
-      notification_level: 1
+      notification_level: level
     }
   });
 };
 
-
 moduleForComponent('topic-notifications-button', { integration: true });
 
 componentTest('the header has a localized title', {
-  template: '{{topic-notifications-button topic=topic}}',
+  template: '{{topic-notifications-button notificationLevel=topic.details.notification_level topic=topic}}',
 
   beforeEach() {
-    this.set("topic", buildTopic());
+    this.set("topic", buildTopic(1));
   },
 
   test(assert) {
     andThen(() => {
-      assert.equal(selectBox().header.name(), "Normal", "it has the correct title");
+      assert.equal(selectKit().header.name(), "Normal", "it has the correct title");
     });
 
+    this.set("topic", buildTopic(2));
+
     andThen(() => {
-      this.set("topic.details.notification_level", 2);
-      assert.equal(selectBox().header.name(), "Tracking", "it correctly changes the title");
+      assert.equal(selectKit().header.name(), "Tracking", "it correctly changes the title");
     });
   }
 });