Make renpy.get_widget() accept a ScreenDisplayable.

This makes it possible to do:

    self.screen = renpy.current_screen()

and later:

    renpy.get_widget(self.current_screen, "name")
Fix XScrollValue and YScrollValue.

These have been broken since we started deferring get_adjustment()s.
sl2: Ensure exceptions in screens work.

This means clearing out the various gui stacks if an exception occurs.
Fix dump of non-ASCII defines.