nautilusgtkplacesview: disconnect from server list monitor changes on destroy

It might happen otherwise that a change is recorded in between the
widget dispose and finalization, causing a crash when setting
the visible name for the GtkStack (as that will be NULL at that point)

Fixes #140
1 job for nautilusgtkplacesview-finalization-fixes in 8 minutes and 5 seconds (queued for 2 seconds)