* `Plugins.plug` now takes a type and property, consumer connects signals.
* Generation of a GIR file.
* Preparation for built-in extensions in `extensions` folder.
* Tweaks to `Database` to avoid exposing `Sqlite` namespace in public API.
Note: Avoiding `owned get; construct;` with `Activatable` interfaces as
used in the definition of `Peas.Activatable` because it triggers a lot
of internal compiler assertions at build time.
For scripting purposes `-execute`, or `-e` for short, exposes actions from the browser window on the command line. `--help-execute` accordingly lists the known actions.