m-permissions-portal: Avoid race condition during shutdown

Attempts to workaround a race condition between daemon thread and
GDBus worker thread during shutdown.

Ubuntu bug: https://bugs.launchpad.net/bugs/2127049

I've not been able to get a symbolic backtrace yet or reproduce it
myself, but the behaviour points to a threading bug. Hypothesis,

Main thread (1, daemon thread) shuts down, unregistering its plugins.
One of the plugins, module-permissions-portal, is triggered to
shutdown.
It tries to clear its GDBus connection handle without disconnecting
its signal handlers.
GDBus thread (2) is in the middle of writing a message on the same
connection handle.
Once finished, it also tries to clear its handle.
The main thread has already taken the signal lock and the signal
handler table ends up in an invalid state, triggering the assert.

I believe this could happen since
wp_portal_permissionstore_plugin_disable is not disconnecting its
signal handlers before trying to clear its DBus object.

See https://bugzilla.gnome.org/show_bug.cgi?id=730296 for more
discussion about this assert in the Glib signal handling code.

modules/module-portal-permissionstore.c

@@ -204,6 +204,8 @@ wp_portal_permissionstore_plugin_disable (WpPlugin * plugin)
       WP_PORTAL_PERMISSIONSTORE_PLUGIN (plugin);
 
   clear_signal (self);
+  if (self->dbus)
+    g_signal_handlers_disconnect_by_data (self->dbus, self);
   g_clear_object (&self->dbus);
 
   wp_object_update_features (WP_OBJECT (self), 0, WP_PLUGIN_FEATURE_ENABLED);