exec_command stdout and stderr

Command executed by self.core.exec_command in plugins should redirect stdout and stderr of the command to the information buffer.

Attached is an example with the link plugin which display the obnoxious warning the firefox process returns.
Here the stderr of the firefox command is instead display in the middle of the poezio UI shifting everything away.

• Changesets:
  • Revision 6698ed80 on 2014-06-26T15:45:05Z:

Redirect stderr to /dev/null when executing a command

fix #2545

• Uploads:
  • 
  • poezio-2545.patch
  • poezio-2545-bis.patch