Fix some leftover references to up. See #482.

• Fixed some docstrings that still mentioned minus/reverse from when reference_up was called gravity.
• Fixed that ob.world.reference_up was initially a tuple (its an array everywhere else).
• Fixed that setting ob.world.reference_up was undone when the local transform changed (the change in world.reference_up was not propagated to the local transform.
• Added a test to make sure the above stays working, based using camera.show_pos.
• Prefixed two callback functions with an underscore.
• Update all (I think) cases that still used ob.up instead of ob.world.reference_up.