Since libpeas doesn't allow a single plugin instance to have multiple
extensions, define a single libpeas extension as a base class for all
plugins and define extensions as non-libpeas interfaces shared by a
single plugin. Manage loading/unloading these ourselves.
This also defines a new trusted extension interface for plugins that
need access to Geary's internals, new error domain for plugis, and
made the notification context a plugin interface that is implemented by
the application object.
