Fix the filter model hack

The current code leads to crashes when there are no matches.
Using weak references to update the filter_model pointer when
that model goes away avoids theses.

https://bugzilla.gnome.org/show_bug.cgi?id=739899
Locations: remove non-existing weather station

There is no weather station in Nitra, Slovakia, since 2012.

https://bugzilla.gnome.org/show_bug.cgi?id=740932
Locations: fix ICAO code of Jinan, China.

The code reported in our sources was wrong.

https://bugzilla.gnome.org/show_bug.cgi?id=721446
Release 3.14.2