The display store is a small container object meant to track
currently known about displays.

It has two signals, "display-added" and "display-removed" that
are supposed to get emitted any time a display gets added or
removed from the store.

Likewise, the GdmManager object has two similar signals that
are supposed to be emitted under similar circumstances.

These signals in GdmDisplayStore and GdmManager were never
actually hooked up to fire at the appropriate times.

This commit changes GdmDisplayStore and GdmManager to properly
fire these signals.