Widget 2.0: newButton: Specifying defaultFrame without overFrame renders button invisible

overFile is now optional. The same should be true for overFrame when using an image sheet.

When creating a button from an image sheet with defaultFrame only, the button becomes invisible.
No errors are logged in the terminal.

Case 21317 [import]uid: 70847 topic_id: 36491 reply_id: 336491[/import]