Add bash completion for gio tool

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)...