Hard coded the indexes for the application tabs.

`indexOfObject` was returning invalid values so the tab would never be selected.
Removed show[Name]Tab methods and replaced with `showTabForIndex` method.

Renamed tab index constants to use `k` naming convention and made the publicly accessible.
Moved placement of constant declarations in WordPressAppDelegate.h
Merge pull request #2041 from wordpress-mobile/issue/1767-open-push-to-notifications

Open push to notifications tab