gdbus-tool: Make --dest optional for emit again

Commit faf94409 made the bash completion more
robust, but in doing so it made the optional --dest argument to `gdbus emit'
mandatory by mistake.

Remove the error case when --dest is not specified. To keep the completion
working, we shuffle the cases around. --dest should be offered up for
completion after --session/--system/--address have been supplied, so we can
complete its argument. Additionally, if --dest isn't specified then we can't
complete --object-path or --signal, so guard these completions accordingly.

https://bugzilla.gnome.org/show_bug.cgi?id=793597