nmtui: some hard-coded Newt-related colors cannot be changed via environment variable

Newt allows to completely change the colors of widgets using environment variables: either by specifying colors directly in NEWT_COLORS or by referring to a file in NEWT_COLORS_FILE. For example:

#!/bin/sh

NEWT_COLORS='
  window=white,red
  border=white,red
  checkbox=white,red
  actcheckbox=white,green
' nmtui

Unfortunately, nmtui cannot be completely customized due to a few hard-coded colors in the source code of nmt_newt_init() (lines 116 to 120):

newtSetColor(NEWT_COLORSET_CHECKBOX, "black", "lightgray");
newtSetColor(NMT_NEWT_COLORSET_BAD_LABEL, "red", "lightgray");
newtSetColor(NMT_NEWT_COLORSET_PLAIN_LABEL, "black", "lightgray");
newtSetColor(NMT_NEWT_COLORSET_DISABLED_BUTTON, "blue", "lightgray");
newtSetColor(NMT_NEWT_COLORSET_TEXTBOX_WITH_BACKGROUND, "black", "white");

(It can be seen when running the above extreme example that checkbox colors do not change indeed.)

Please remove forcing the NEWT_COLORSET_CHECKBOX color and make it possible to customize the extra NMT_NEWT_COLORSET_<WIDGET> colors via some environment variable.

Thanks for the great piece of software!