add absolute path of Python executable to restart command

otherwise, Frescobaldi would always be restarted using /usr/bin/python
as the Python executable, instead of the actual executable used to start
it the first time

(e.g. on Mac OS X the most convenient way to start Frescobaldi is using MacPorts to manage its dependencies; this means that the Python executable provided by MacPorts must be used, and not the one provided by the system in /usr/bin/python)