Added support for user choosen terminal, which fixes #bgo737088

With this commit the "terminal" entry in the configuration file got
obsolete. Instead of the last one, two new variables are available now:
"terminal_open" and "terminal_exec". Their default values are set in
src/gnome-cmd-data.cc. They can be adjusted at runrime in
"Settings->Options->Programs".
Updated NEWS and releases.xml