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-footer-mobile-dropdown-test.js.es6

@@ -17,16 +17,16 @@ componentTest('default', {
   },
 
   test(assert) {
-    expandSelectBoxKit();
+    expandSelectKit();
 
     andThen(() => {
-      assert.equal(selectBox().header.name(), "Topic Controls");
-      assert.equal(selectBox().rowByIndex(0).name(), "Bookmark");
-      assert.equal(selectBox().rowByIndex(1).name(), "Share");
-      assert.equal(selectBox().selectedRow.el.length, 0, "it doesn’t preselect first row");
+      assert.equal(selectKit().header.name(), "Topic Controls");
+      assert.equal(selectKit().rowByIndex(0).name(), "Bookmark");
+      assert.equal(selectKit().rowByIndex(1).name(), "Share");
+      assert.equal(selectKit().selectedRow.el.length, 0, "it doesn’t preselect first row");
     });
 
-    selectBoxKitSelectRow("share");
+    selectKitSelectRow("share");
 
     andThen(() => {
       assert.equal(this.get("value"), null, "it resets the value");