misc: add objectManager class

The D-Bus ObjectManager interface is fairly recent addition to the
D-Bus specification. Its purpose is to provide a standardized way
to track objects dynamically coming and going for a service, and
to track capabilities dynamically coming and going for those objects
(by means of interfaces).

This commit adds the requisite code needed to make use of the
ObjectManager interface.

It will ultimately be needed to implement smartcard support in the
login screen.

https://bugzilla.gnome.org/show_bug.cgi?id=683437