OrientationUI: Allow user to specify prim var name for Houdini's quaternion.
OrientationUI: Allow user to specify prim var name for Houdini's quaternion.

Houdini convention is to write quaternion for orientation in the `orient`
prim var. Without allowing prim var name selection, the node is hard to use
with Alembic files coming from Houdini.

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