refactor(button): add ion-button attribute and icon attributes to style buttons

BREAKING CHANGES:

- `<button>` becomes `<button ion-button>`
- `<a button>` becomes `<a ion-button>`
- `<button ion-item>` does not get the `ion-button` attribute
- Buttons inside of `<ion-item-options>` do get the `ion-button`
attribute
- Removed the `category` attribute, this should be passed in
`ion-button` instead.
- Button attributes added for icons in buttons: `icon-only`,
`icon-left`, and `icon-right`

closes #7466

src/components/checkbox/checkbox.ts

@@ -53,14 +53,13 @@ export const CHECKBOX_VALUE_ACCESSOR = new Provider(
     <div class="checkbox-icon" [class.checkbox-checked]="_checked">
       <div class="checkbox-inner"></div>
     </div>
-    <button role="checkbox"
+    <button ion-button="item-cover"
+            role="checkbox"
             type="button"
-            category="item-cover"
             [id]="id"
             [attr.aria-checked]="_checked"
             [attr.aria-labelledby]="_labelId"
-            [attr.aria-disabled]="_disabled"
-            class="item-cover">
+            [attr.aria-disabled]="_disabled">
     </button>
   `,
   host: {