BUG #14452: Fixed GoToLast() in Python.

Functions in simple.py that relied on "singleton" proxies were not consistently
finding those proxies and ended up creating duplicate ones on occassion. This
was causing methods to use wrong animation scenes, for example, resulting in
things not working exactly right. Fixed that by adding function (_find_proxy) to
use to locate such singleton properties and ensuring that that function works
correctly.

Change-Id: Ia7795ff78e271e83655f964c70605c65d86b4708
Merge topic '14452_fix_go_to_last' into next

614e786 BUG #14452: Fixed GoToLast() in Python.