MT.D usability improvements

[akcoder]

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!

Updated to comply with public naming rules.