Devoncarew editor types

Add a generic way for content specific editors to customize their `ace.EditSession` instance. Remove some older style content specific customization in `AceManager`. Make sure that specific content types have their correct handling for hard / soft tabs. Also, remove an unnecessary throw / catch of an exception on startup.

@umop
Why the duplicate override (from this line: https://github.com/dart-lang/chromedeveditor/pull/2735/files#diff-0b5e32d71f397a282e3fb4512c967813R198 )?