main: allow session mode to be specified in the environment

Specifying the session mode on the command-line doesn't play
well with session management (since the saved session desktop
file well either drop the specified session mode, or force it
always, even if the user picked a different mode at the login
screen)

This commit adds support for specifying the session mode via an
enviroment variable. For now, keep the old command line interface
for backward compatibility

https://bugzilla.gnome.org/show_bug.cgi?id=720894
Updated Spanish translation