MT.D usability improvements

Few improvements to MT.D,
 * Make searchBar protected in DialogViewController - needed so I can clear the search bar when the view is being navigated away from
 * Make PerformFilter virtual in DialogViewController - needed so I can override it
 * Make IndexOf public in Element
 * Fix the doc comment for Insert in Element
 * Simplify the Remove logic in Element
 * Add overloads of Remove that take in UITableViewRowAnimation

Thank you for the great work! By switching to MT.D, I was able to eliminate a bunch of cruft classes and vastly simplify the code!