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()
```

plugins/poll/test/javascripts/widgets/discourse-poll-option-test.js.es6

@@ -14,7 +14,7 @@ widgetTest('single, not selected', {
   },
 
   test(assert) {
-    assert.ok(find('li .d-icon-circle-o:eq(0)').length === 1);
+    assert.ok(find('li .d-circle-o:eq(0)').length === 1);
   }
 });
 
@@ -27,7 +27,7 @@ widgetTest('single, selected', {
   },
 
   test(assert) {
-    assert.ok(find('li .d-icon-dot-circle-o:eq(0)').length === 1);
+    assert.ok(find('li .d-dot-circle-o:eq(0)').length === 1);
   }
 });
 
@@ -43,7 +43,7 @@ widgetTest('multi, not selected', {
   },
 
   test(assert) {
-    assert.ok(find('li .d-icon-square-o:eq(0)').length === 1);
+    assert.ok(find('li .d-square-o:eq(0)').length === 1);
   }
 });
 
@@ -59,6 +59,6 @@ widgetTest('multi, selected', {
   },
 
   test(assert) {
-    assert.ok(find('li .d-icon-check-square-o:eq(0)').length === 1);
+    assert.ok(find('li .d-check-square-o:eq(0)').length === 1);
   }
 });