GVfs utils used to have bash completion, which was pretty useful. However,
it hasn't been ported to gio tool unfortunately. GLib provides completion
for various utils already, so it would be nice to provide completion also
for gio tool. I've updated old bash completion code and merged with some
my old unmerged fixes.

The gvfs completion used "gvfs-ls --show-completions" helper. This mentioned
option hasn't been obviously ported to "gio list" and the proposed completion
doesn't add this option in "gio list" to not pollute the codes, but maybe it
is a bit slower as consequence.

The proposed bash completion suggests subcommands, uris and paths including
the remote mounts. It contains some workarounds, especially because of proper
handling of paths with colons and other special chars (like spaces)...