Added select method to tabbar elements so it can be manually selected.

I've added a method to tabbar elements so it can be manually selected.

Eg:

 ```javascript
document.getElementsByTagName('x-tabbar-tab')[0].select()
 ```
This method marks the specified tab as 'selected' and displays the target-element for the tabbar (using the method _onTabbarTabTap).

The method _selectTab() was extracted from the tap event from x-tabbar so I replace it current implementation to this method.