app: add base classes for the extension manager.

app: add base classes for the extension manager.

Right now it only loads AppStream data, which is completely useless, yet
is a base of a managed extension system. Having proper metadata is what
will allow to actually know what is installed.
This is only the first draft.

Note that I am not adding the extension path into GimpCoreConfig on
purpose, since the point is not to have people manage their extension
directories manually anymore.
The extensions will be loaded from the build-time system path or the
config directory, and that's all.
What will probably be stored in the config though will be the remote
repositories URLs (allowing third-party extension repositories).