utils.AsyncCommand: fix missing  environment.

Add default environment to glib.spawn_async.
zim,commands: prevent crashing on AsyncCommand

AsyncCommand object is keep in Action - this prevent losing reference to it
and remove object.
keyring: support python-keyring 3+

Based on: patch for Debian #728468 by Sebastian Ramacher
firefox: fix loading tags
plugin_support: fix importing keyrings

+ try to importy GnomeKeyring from keyring.backends.Gnome
gnome-keyring now work again

As long using keyring.backend.GnomeKeyring require gi.repository and
rewrite all gnome dependency, support for gnome-keyring is provided direcly
by gnomekeyring module.