Create a DEFAULT_ALTITUDE macro

It is set to 280.0, which is roughly the average altitude of dry land
Bind signal callbacks with gtk_widget_class_bind_template_callback()
Fix ag_db_get_chart_data_by_id() to recognise NULL values
Read country/city list from geodata.xml into GtkListStores
Update UI to use country/city chooser
Empty/update city and country fields