Nautilusgtkplacesview finalization fixes

As gtk!119 (merged)

When destroying this widget we could have this warning:

GLib-CRITICAL **: 23:06:12.109: Source ID 1925 was not found when attempting to remove it

Or a crash (or warning once gtk!117 (merged) is merged) because populate_servers could be called in between the disposition and finalization (see ubuntu bug, fedora bug or #140 (closed)).