app: new "gex" format (GIMP Extension).
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).
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