GafferUI : Support alternate shiboken install location.
GafferUI : Support alternate shiboken install location.

If PySide has been compiled with an embedded shiboken, we should use that one. Otherwise fall back to an external shiboken.

appleseedhq/gaffer,chippey/gaffer,GafferHQ/gaffer,chippey/gaffer,lucienfostier/gaffer,andrewkaufman/gaffer,hradec/gaffer,boberfly/gaffer,andrewkaufman/gaffer,ivanimanishi/gaffer,boberfly/gaffer,ImageEngine/gaffer,ImageEngine/gaffer,johnhaddon/gaffer,ImageEngine/gaffer,ivanimanishi/gaffer,hradec/gaffer,johnhaddon/gaffer,GafferHQ/gaffer,lucienfostier/gaffer,andrewkaufman/gaffer,GafferHQ/gaffer,ivanimanishi/gaffer,appleseedhq/gaffer,appleseedhq/gaffer,appleseedhq/gaffer,chippey/gaffer,lucienfostier/gaffer,lucienfostier/gaffer,andrewkaufman/gaffer,ivanimanishi/gaffer,hradec/gaffer,boberfly/gaffer,hradec/gaffer,chippey/gaffer,andrewkaufman/gaffer,hradec/gaffer,johnhaddon/gaffer,GafferHQ/gaffer,johnhaddon/gaffer,johnhaddon/gaffer,boberfly/gaffer,ImageEngine/gaffer,GafferHQ/gaffer