Match OSC reply sequence terminator to OSC request terminator.

Traditionally konsole only was able to parse OSC sequences terminated with
non-standard BEL. In 324d78c0 konsole was extended to parse sequences
terminated by 7-bit ST (ESC \) as well. But the OSC responses were still
hardcoded to reply only with sequences with BEL. But replying to 7-bit ST
terminated sequences with a reply terminated by a non-standard terminator
is wrong.

This commit passes the terminator along and changes the response to match
the original request.

printf "\033]10;?\a" replies with "^[]10;rgb:fcfc/fcfc/fcfc^G"
printf "\033]10;?\033\\" replies with "^[]10;rgb:fcfc/fcfc/fcfc^[\"