Message tray sources cannot be reused after destruction, so connect
to 'destroy' signal and clear out the previous one.
Also, fix some code paths that used the autorun manager incorrectly.
https://bugzilla.gnome.org/show_bug.cgi?id=683377
Previous code was activating the networkAgent and telepathyClient
in the lock-screen, irrespective of the previous mode.
Now it checks if the session mode is locked down, and if so it refuses
to start new components.
https://bugzilla.gnome.org/show_bug.cgi?id=683400
Components are pieces of the shell code that can be added/removed
at runtime, like extension, but are tied more directly to a session
mode. The session polkit agent, the network agent, autorun/automount,
are all components, keyring, recorder and telepathy client are all
now copmonents.
https://bugzilla.gnome.org/show_bug.cgi?id=683156
