Typos and I fleshed out the self-defense mechanisms of the
Picker callbacks.  They'll only accept changes now if the
panel is in Edit or Add mode, and has an edited object in
place.  Otherwise the callback does nothing, preventing data
corruption if someone had opened a Picker, ended the edit,
and somehow not closed the window at the same time (maybe do
to an error; I don't know.  Something.  It could have happened...
*whinge*)