gio: Add hack to close up dbus-daemon race

If gio open exits before the program it starts fully activates, then
the dbus-daemon may avoid doing the activating method call.

This commit works around the problem by pinging the activated application,
and waiting for a reply.

Same workaround is used in gtk-launch and was used in gvfs-open before
it was replaced by gio open.

https://bugzilla.gnome.org/show_bug.cgi?id=780296