misc: add code to use settings-daemon smartcard service

gnome-settings-daemon monitors smartcard insertion and removal
events on the system and then exports a model of the current
smartcard topology over the bus using the D-Bus ObjectManager interface.

This commit adds the support code needed in gnome-shell to talk to
the gnome-settings-daemon service.

A future commit will use this code to inform the login screen
when a user inserts a smartcard (so it can react appropriately)

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