Add "sealed" key word to mark some public/protected methods not be overriden

From #1524, I see that the default method are all "virtual", but sometimes I don't wanna some public method to be overidden. Would you invent a mechasim to force the users that they cannot override a public method (with the key word, something like "sealed")?

Thanks!

[DavidANeil]

Please add a final or sealed accessibility modifier. To be to public as private is to protected.

	Overridable	Non-overridable
Internal	protected	private
All	public	final

[jonlepage]

Has this fallen into oblivion?
This is an essential features for the sugars design of an public extensible API!