added a proper register-sign API call

added a proper register-sign API call
(see api.lua and standard_signs.lua for examples)

make selectionbox call now ignores the old onpole flag (it's automatic now)
removed debug print()'s
moved the signs-on-fences lbm settings to here, from basic_signs.
some misc cleanups

added reference models for standard wooden sign ("wallmounted" on wall,
"facedir" on wall, and pole-mount variants, using standard-size pole
mount), and a separate .blend with street_signs' OM3 sign on slim pole
mount.