Make defaultbutton|cancelbutton optional

[form-follows-function]

defaultbutton and cancelbutton are not always needed in a GUI since you can create custom buttons and cancel the dialog by either clicking the window or quitting the app.

Example:
Embedding custom buttons to have a user select an option doesn't require a confirmation button for the dialog.

[form-follows-function]

Additional idea:
Default button should only appear if no other button is defined