Bind GSetting key to D-Bus property

Bind GSetting key to D-Bus property

Modifications made to luc-handler/luc-handler-service.c
In struct _LUCHandlerService:
- Added GSettings object to store the settings with the handler.

In luc_handler_service_init:
- Initialized the GSettings object.
- Bound the last-user-context of the settings to the last-user-context
  of the interface.

In luc_handler_service_finalize:
- Finalize the GSettings object (It is unnecessary to unbind the settings
  because they belong to the object they are bound to, and are unbound
  when the object is finalized)