Add support for VT100 terminal foreground/background color query

[FabriceSalvaire]

Sometimes, it can be useful to know if the user is using a black or white terminal to set colours.

I got this Bash script, but I am unable to make it works on Python:

#!/bin/bash
# /dev//tty is the terminal for the current process
success=false
# redirect shell stdin to tty
# useless ???
# exec < /dev/tty
# -g --save
#   return a string
oldstty=$(stty -g)
# raw same as
#     -ignbrk -brkint -ignpar -parmrk -inpck -istrip -inlcr -igncr -icrnl -ixon -ixoff -icanon -opost  -isig
#     -iuclc -ixany -imaxbel -xcase min 1 time 0
# -echo   NO echo input characters
# min N   with -icanon, set N characters minimum for a completed read
# useless ???
# stty raw -echo min 0
stty -echo
# else
#  ^[]11;rgb:2323/2626/2727^[\rgb:2323/2626/2727
col=11   # background
# -n   do not append a newline
# -e   enable interpretation of the following backslash escapes
#          OSC   Ps  ;Pt ST
echo -en "\033]${col};?\033\\" > /dev/tty  # echo opts differ w/ OSes
# 10;rgb:fcfc/fcfc/fcfc
# 11;rgb:2323/2626/2727
result=
# IFS Input Field Separator
# Read a line from the standard input and split it into fields.
#   -r         do not allow backslashes to escape any characters
#   -d delim   continue until the first character of DELIM is read, rather than newline
# useless ???
# if IFS=';' read -r -d '\' color ; then
if read -r -d '\' color ; then
    # s/^.*\;// ; s/[^rgb:0-9a-f/]//g
    result=$(echo $color | sed 's/^.*\;//')
    success=true
fi
# restore tty setup
stty $oldstty
# print value
echo $result
# rgb:2323/2626/2727
$success

The magic seems in stty -echo to disable echo and read -r -d '\' color to get the value.

I didn't find any code in prompt_toolkit to query VT-100 (excepted cursor position ???).