action: improve printing

action: improve printing

The 'value' field of a ModelAction might actually represent a signed
number, a pointer, or something else entirely, so values look very huge
and uninformative when printed, especially in decimal. Hexadecimal
representation makes everything a little more readable, I think.

While I'm at it, I adjust the spacing a little and make VALUE_NONE into
a different magic number which is more recognizable in hex.