Completely new experimental Autostart feature.

The Autostart feature allows users to create their own set of sessions, each having
their own set of terminals, and letting them be spawned on startup, automatically.

Each terminal can have multiple properties:
    * Name;
    * Profile (not yet implemented);
    * Neighbor: a terminal to split against;
    * Split way: Left->Right or Top->Bottom.
    * Commands: a list of shell commands to be run after spawn.

Not yet supported:
    * Session save and restore instead of manual definition;
    * Profile definition as stated above.

The Autostart feature doesn't break previous user configurations and has been
designed to be extensible without ever breaking user configurations.