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/segment/test/basic/main.html

@@ -13,7 +13,7 @@
 
   <ion-toolbar no-border-top>
     <ion-buttons end>
-      <button>
+      <button ion-button icon-only>
         <ion-icon name="search"></ion-icon>
       </button>
     </ion-buttons>
@@ -96,7 +96,7 @@
     </ion-segment-button>
   </ion-segment>
 
-  <button block dark (click)="toggleDisabled()">Toggle Disabled</button>
+  <button ion-button block dark (click)="toggleDisabled()">Toggle Disabled</button>
 
 </ion-content>
 

src/components/segment/test/nav/index.ts

@@ -21,7 +21,7 @@ class SegmentPage {
   template: `
     <ion-header>
       <ion-navbar hideBackButton>
-        <button menuToggle>
+        <button ion-button menuToggle>
           <ion-icon name="menu"></ion-icon>
         </button>
         <ion-title>

src/components/segment/test/nav/main.html

@@ -27,13 +27,13 @@
 
   <ion-row>
     <ion-col>
-      <button block (click)="signInType='new'">New</button>
+      <button ion-button block (click)="signInType='new'">New</button>
     </ion-col>
     <ion-col>
-      <button light block (click)="signInType='existing'">Existing</button>
+      <button ion-button light block (click)="signInType='existing'">Existing</button>
     </ion-col>
     <ion-col>
-      <button dark block (click)="signInType='test'" class="e2eSegmentTestButton">Test</button>
+      <button ion-button dark block (click)="signInType='test'" class="e2eSegmentTestButton">Test</button>
     </ion-col>
   </ion-row>
 
@@ -49,6 +49,6 @@
     </div>
  </div>
 
- <button block (click)="goToPage2()">Next Page</button>
+ <button ion-button block (click)="goToPage2()">Next Page</button>
 
 </ion-content>