DialogView lacked support for handling a cancel request from the

browser, such as when the user hit the Escape key. The dialog would

close with no feedback. This was a problem for the Confirm dialog,

which never went through the cancel response flow.
There's now explicit support for handling this and direct closing using

the cancel and close events, along with new Cancel actions.
When closing a dialog, or when receiving a cancel/close request, a Close

Reason will be set. This contains a flag indicating if the dialog was

canceled, along with optional fields for the action triggering the

closure and any state returned from the action's click handler.
To support that on the action side, we needed a Cancel action type. To

avoid introducing yet type requiring a Callback variant, we're

deprecating the *_CALLBACK versions of the types. These weren't really

necessary, as they just determined whether to invoke the provided

callback= argument.
We now support CLOSE and CANCEL types, with CALLBACK and

CALLBACK_AND_CLOSE being deprecated. A default of null is equivalent

to CALLBACK, which invokes an action but does not close the dialog.

The old types will convert to the new on load, but are not formally

deprecated for now.
If the cancel event is triggered on the element, and there's a

Cancel action, it will be invoked directly, allowing any callback to

be run.

Tested each action type, new and old, and verified the right behavior.

Tested both in Review Board and with the new story updates.
Tested that hitting Escape closed the Confirm dialog, and triggered

the Cancel dialog behavior.