Another fix for Terminal=true

A terminal emulator is launched if .desktop file contains "Terminal=true" statement and there is no control terminal assigned to our process. The terminal emulator can be set via $TERMINAL environment variable that should contain either its name or a command line to launch the terminal. In case this variable is unset, xdg-open iterates through a number of pre-known terminal apps, starting with the Debian specific x-terminal-emulator binary.

Fixes: #84
See also: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=964877