When we allow users to choose between different templates, we should
provide some context for each template to facilitate that choice.
Add that metadata in the form of a .desktop file, which allows us to
specify name and description, and is well supported by our translation
infrastructure.
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/812
The template used when creating a new extension is intentionally
minimal, as the sample code in the old extensions-tool proved to
be annoying more often than not.
However as we support per-command options, we don't have to limit
ourselves to a single template, and can offer alternatives for
common use cases.
To prepare for that, namespace the existing template by moving it
into a subfolder.
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/812
