feat: add MdTextButton to View APIs (#15328)

* view: make button focusable by default

* view: add MdTextButton

* view: add common methods to LabelButton

lib/browser/api/module-list.js

@@ -41,6 +41,7 @@ if (features.isViewApiEnabled()) {
     { name: 'Button', file: 'views/button' },
     { name: 'LabelButton', file: 'views/label-button' },
     { name: 'LayoutManager', file: 'views/layout-manager' },
+    { name: 'MdTextButton', file: 'views/md-text-button' },
     { name: 'TextField', file: 'views/text-field' }
   )
 }