File extension (.gex) may still change if anything better is proposed.
This format is currently just a compressed archive containing the
extension data (plug-in, brushes or whatever) and the metadata file.

For now, opening such file will simply install it as a new extension,
keeping all file permissions and structure. Of course in the future, it
will have to trigger a confirmation dialog.

Currently the compression used is zip, which is just a first draft. This
is not a decisive choice as well. We could use some tarball compressed
in whatever other compression algorithm. I use libarchive as a new
dependency to support unarchiving as it seems to be a common library
(and since it is already used by AppStream-glib anyway, this doesn't add
any actual dependency, just make it direct).