Integrate the slaves into the main daemon process

For no particular reason, the slave has been kept as a separate process.
Integrate this into the main display process by simply making the display
keep a handle to a GdmSlave object. To keep the cleanup simple, we won't
remove the GdmSlave subtypes yet. A future cleanup should merge the slave
functionality into GdmDisplay and its subtypes.

https://bugzilla.gnome.org/show_bug.cgi?id=726380