Allow setting default text color in sign node def

Allow setting default text color in sign node def

Caveat: signs used to reset the text color to black at the start of each
line. Now, they reset to the node's default text color, after which
users' "#x" color codes are applied.  This means some signs will change
to a new color where the user didn't explicitly set it.  This will only
be visible on green, red w/white border, white w/red border, blue, and
brown metal signs, as these have either white or red as the new default
color.  All other signs will render in black, as usual.