Add read-only GListModel wrapper
Many people are starting to use
GListModel, which is great. However, sometimes we use a
GListStore internally and we don't want to allow the consumer to modify the model (but be able to give them access to its contents read-only).
Instead, it might be nice to have a wrapper
GListModel which is simply a read-only variant proxying the signals from the parent. This allows us to ensure the consumer cannot mutate our internal state.