Encode PSS_play extension using UTF-8.

This prevents a crash that occurs when a unicode filename is passed
to PSS_play.

Fixes #369, thanks to everyone for tracking this down.
Transform.set_child should update the transform's children.

This lets us pass events to a new child when the child changes.
Add an 'events' transform property.

This determines if events will be passed to the child of the
transform. This allows one to prevent events from being passed
to the old_widget in an ATL transform.
Transform: Fix and optimize argument parsing.

Transform had a problem parsing arguments with underscores in the
name, like transform_anchor. That's been fixed.

Transform also did a lot of allocation for rare cases, such a
Transform being given an argument with a style prefix,
like hover_xpos. This has been optimized in order to reduce
the amount of allocation required.