pgp: Expose a key's fields using GListModel

The current API mostly used GList (which is a linked list). This worked well enough for most use cases, but becomes a bit cumbersome to use with modern GTK APIs, which favor the adaptive GListModel. As such, this commit now exposes the keys, subkeys and UIDs of a PGP key using a GListModel instead of a GList.

This also required some internal changes for the SeahorseGpgmeKey subclass, which kept a separate copy of UIDs (and had a lot of back and forth of vfunc calling between itself and its parent class). The new class is much simpler and doesn't seem to give any regressions.