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/post-test.js.es6

@@ -436,7 +436,7 @@ widgetTest("reply directly above", {
     click('a.reply-to-tab');
     andThen(() => {
       assert.equal(this.$('section.embedded-posts.top .cooked').length, 1);
-      assert.equal(this.$('section.embedded-posts .d-icon-arrow-up').length, 1);
+      assert.equal(this.$('section.embedded-posts .d-arrow-up').length, 1);
     });
   }
 });
@@ -673,7 +673,7 @@ widgetTest("replies - one below, not suppressed", {
     click('button.show-replies');
     andThen(() => {
       assert.equal(this.$('section.embedded-posts.bottom .cooked').length, 1);
-      assert.equal(this.$('section.embedded-posts .d-icon-arrow-down').length, 1);
+      assert.equal(this.$('section.embedded-posts .d-arrow-down').length, 1);
     });
   }
 });
@@ -760,7 +760,7 @@ widgetTest("topic map - links", {
     click('nav.buttons button');
     andThen(() => {
       assert.equal(this.$('.map.map-collapsed').length, 0);
-      assert.equal(this.$('.topic-map .d-icon-chevron-up').length, 1);
+      assert.equal(this.$('.topic-map .d-chevron-up').length, 1);
       assert.equal(this.$('.topic-map-expanded').length, 1);
       assert.equal(this.$('.topic-map-expanded .topic-link').length, 5, 'it limits the links displayed');
     });