autorun: add an AutorunManager class

AutorunManager is a class that takes care of displaying and managing
notifications and UI for storage devices.

When a mount appears and a number of conditions are satisified, a
transient notification will be displayed to immediately interact with
the device. AutorunTransientDispatcher is the object that takes care of
showing/hiding the notification sources as devices appear/disappear.

Likewise, current mounts are kept in a list and presented within a
list in a resident notification, handled by AutorunResidentSource.

https://bugzilla.gnome.org/show_bug.cgi?id=653520