[libqofono] Restore public ABI

All public methods that were present earlier have been restored. This
doesn't guarantee binary compatibility with the clients that were directly
using private methods but it's their fault.

Objects have the same size as before the ABI break, so even the code that
allocates libqofono objects on stack might work. At least it won't crash.
[libqofono] Added OfonoNetworkOperatorListModel

To simplify operator selection UI.
[libqofono] Fixed signal name for QOfonoConnectionContext 'type' property