otapi: API breakage: the network code will change to string

otapi: API breakage: the network code will change to string

The reason is that the operatorcode consists of MCC and MNC and MNC
can be two or three digits long.
So int 1010 could either mean 010:10 or 001:10 and we can't really
tell...

This affects the following methods and signals:
signal:GSM.Network.Status - ("code", int) -> ("code", string)
method:GSM.Network.GetStatus - same here
method:GSM.Network.ListProviders - a(issss) -> a(sssss)
method:GSM.Network.RegisterWithProvider - i -> s
method:GSM.SIM.GetProviderList - a{is} -> a{ss}