make execInNewTerminalWindow under linux asynchronous

gnome-terminal under linux blocks until it's finished, so it must be
sent to the background

Change-Id: Ica5724c36f48f5f8f6d322da5a99e636a3679bfe
Reviewed-on: http://naan.mimuw.edu.pl:8001/101
Tested-by: Nebulostore Nebulostore <nebulostore@gmail.com>
Reviewed-by: Lukasz Siczek <siczek.lukasz@gmail.com>