Add cancellable argument for dbus calls

The version of PyGObject shipped in Slackware doesn't seem to have
cancellable=None as default argument.
Add argument user_data=None for signal_subscribe()
Add argv parameter to Gtk.init_check
Add user_data parameter for Notify.add_action
Add an unknown argument for older Notify version
Add user_data argument to callback
Merge branch 'fix-slackware-dbus-call' into maint-1.0
Bump version 1.0.4
Merge branch 'maint-1.0'