Add extensibility to the comment dialog.
The comment dialog is now extensible through a CommentDialogHook. Such

hooks provide a view that will essentially layer onto the CommentDialog

and can manipulate or listen to it or its associated CommentEditor.
Some of the internals of CommentDialog have been made public for use by

extensions.
Right now, hooks can manipulate the buttons and trigger operations such

as saving. Later, we'll probably want to allow it to add additional

fields.

Tested with an extension I'm building, and tested without. Unit tests also pass.