FIX: adds select-kit api to modify header computed content (#5476)

This api would allow to simply modify header text or icons, eg:

```
      api.modifySelectKit("select-kit")
        .modifyHeaderComputedContent((context, computedContent) => {
          computedContent.title = "Not so evil";
          return computedContent;
        });
```

test/javascripts/components/single-select-test.js.es6

@@ -103,7 +103,7 @@ componentTest('custom search icon', {
 
     andThen(() => {
       assert.ok(
-        this.get('subject').filter().icon().hasClass("fa-shower"),
+        this.get('subject').filter().icon().hasClass("d-icon-shower"),
         "it has a the correct icon"
       );
     });
@@ -532,3 +532,28 @@ componentTest('with title', {
     andThen(() => assert.equal(this.get('subject').header().title(), 'My title') );
   }
 });
+
+
+componentTest('support modifying header computed content through plugin api', {
+  template: '{{single-select content=content}}',
+
+  beforeEach() {
+    withPluginApi('0.8.15', api => {
+      api.modifySelectKit("select-kit")
+        .modifyHeaderComputedContent((context, computedContent) => {
+          computedContent.title = "Not so evil";
+          return computedContent;
+        });
+    });
+
+    this.set("content", [{ id: "1", name: "robin"}]);
+  },
+
+  test(assert) {
+    andThen(() => {
+      assert.equal(this.get('subject').header().title(), "Not so evil");
+    });
+
+    andThen(() => clearCallbacks());
+  }
+});