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/input/input.ts

@@ -78,7 +78,7 @@ import { Platform } from '../../platform/platform';
   template: `
     <input [type]="type" [(ngModel)]="_value" (blur)="inputBlurred($event)" (focus)="inputFocused($event)" [placeholder]="placeholder" class="text-input">
     <input [type]="type" aria-hidden="true" next-input *ngIf="_useAssist">
-    <button clear [hidden]="!clearInput" type="button" class="text-input-clear-icon" (click)="clearTextInput()" (mousedown)="clearTextInput()"></button>
+    <button ion-button clear [hidden]="!clearInput" type="button" class="text-input-clear-icon" (click)="clearTextInput()" (mousedown)="clearTextInput()"></button>
     <div (touchstart)="pointerStart($event)" (touchend)="pointerEnd($event)" (mousedown)="pointerStart($event)" (mouseup)="pointerEnd($event)" class="input-cover" tappable *ngIf="_useAssist"></div>
   `,
   directives: [NativeInput, NextInput, NgIf, NgModel],