feat(list): add list headers and item dividers as items

- `ion-list-header` and `ion-item-divider` now take advantage of the
same content projection as the `ion-item`
- they are still styled as list headers and item dividers
- added docs and demos on this addition

closes #5561

src/components/input/test/clear-input/index.ts

@@ -7,6 +7,10 @@ import {ionicBootstrap} from '../../../../../src';
 })
 class E2EPage {
   myValue = 'value';
+
+  clicked() {
+    console.log("clicked button");
+  }
 }
 
 @Component({

src/components/input/test/clear-input/main.html

@@ -12,10 +12,16 @@
   <ion-list>
 
     <ion-item>
-      <ion-label>Text 1:</ion-label>
+      <ion-label>Input Clear:</ion-label>
       <ion-input class="e2eClearInput" [(ngModel)]="myValue" clearInput></ion-input>
     </ion-item>
 
+    <ion-item>
+      <ion-label>Button:</ion-label>
+      <ion-input class="e2eClearInput" [(ngModel)]="myValue"></ion-input>
+      <button item-right (click)="clicked()">Button</button>
+    </ion-item>
+
   </ion-list>
 
 </ion-content>