combobox should be replaced by a gtktreeview for city selection

[Frédéric Crozat]

for countries where there is a lot of different cities covered for DVB (currently, France has 111, Finland 188), combobox is not a good widget to choose from. A gtktreeview should be used instead with a gtkliststore (not for all cities from all countries, since it would require too much time to parse all files)