automount: add an AutomountManager class

The AutomountManager class is the low-level counterpart of the
previously introduced AutorunManager, and takes care of extracting the
list of valid mounts from a GVolume or a GDrive and mounting them,
provided a number of conditions and requirements are met.

AutomountManager also keeps track of the current session availability
(using the ConsoleKit and gnome-screensaver DBus interfaces) and
inhibits mounting if the current session is locked, or another session
is in use instead.

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