refactor(colors): color should be added as an input instead of directly adding the color to the component

BREAKING CHANGES:

Colors should be passed in the `color` input on components, not added
individually as an attribute on the component.

For example:

```
<ion-tabs primary>
```

Becomes

```
<ion-tabs color=”primary”>
```

Or to bind an expression to color:

```
<ion-navbar [color]="barColor">
   ...
</ion-navbar>
```

```ts
@Component({
  templateUrl: 'build/pages/about/about.html'
})
export class AboutPage {
  barColor: string;

  constructor(private nav: NavController, platform: Platform) {
    this.barColor = platform.is('android') ? 'primary' : 'light';
  }
}
```

Reason for this change:
It was difficult to dynamically add colors to components, especially if
the name of the color attribute was unknown in the template.
This change keeps the css flat since we aren’t chaining color
attributes on components and instead we assign a class to the component
which includes the color’s name.
This allows you to easily toggle a component between multiple colors.
Speeds up performance because we are no longer reading through all of
the attributes to grab the color ones.

references #7467
closes #7087 closes #7401 closes #7523

src/components/label/label.ts

@@ -56,6 +56,21 @@ import { Attribute, Directive, ElementRef, Renderer, Input } from '@angular/core
 })
 export class Label {
   private _id: string;
+  
+  /** @internal */ 
+  _color: string;
+
+  /**
+   * @input {string} The predefined color to use. For example: `"primary"`, `"secondary"`, `"danger"`.
+   */
+  @Input()
+  get color(): string {
+    return this._color;
+  }
+
+  set color(value: string) {
+    this._updateColor(value);
+  }
 
   /**
    * @private
@@ -103,4 +118,22 @@ export class Label {
     this._renderer.setElementClass(this._elementRef.nativeElement, className, true);
   }
 
+  /**
+   * @internal
+   */
+  _updateColor(newColor: string) {
+    this._setElementColor(this._color, false);
+    this._setElementColor(newColor, true);
+    this._color = newColor;
+  }
+
+  /**
+   * @internal
+   */
+  _setElementColor(color: string, isAdd: boolean) {
+    if (color !== null && color !== '') {
+      this._renderer.setElementClass(this._elementRef.nativeElement, `label-${color}`, isAdd);
+    }
+  }
+
 }