Look first in the PySide package for Qt's plugins folder,

instead of just in Qt's install or build folder.  Only do it if the path
exists in the PySide package, and if it's not already on the search path.

Change-Id: I5a8ec8724cad2cb011e7e1c364c82a8b87289c5c
Reviewed-by: John Ehresman <jpe@wingware.com>