gtkplacesview: 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)
1 job for gtkplacesview-finalization-fixes in 7 minutes and 31 seconds (queued for 3 seconds)
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passed build #25233

00:07:31