* Prompters have state, and cancelling an already prompter will
cause prompts that are in progress to fail.
* In addition allow replacement of our shell prompter for debugging
purposes.
https://bugzilla.gnome.org/show_bug.cgi?id=695485
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