red-brick: More work on the New Program wizard

Add combo boxes to pick .jar and .py files.

Refactor argument list editing to a ListWidgetEditor helper class.
red-brick/npw: Ensure that files can only be added once
red-brick/npw: Fix method name style of emitCompleteChanged
red-brick/npw: Use more constants
red-brick/npw: Fix schedule now timestamp value
red-brick/npw: Add UI logic for stdio page