applet: fix wrong status for wireless toggle

When "state-changed" signal emits, there's a chance that wireless status has't
update yet, so the "Enable Wi-Fi" toggle shows the wrong status.

This can be solved by updating applet menu whenever "wireless-enabled" changes.

https://mail.gnome.org/archives/networkmanager-list/2016-August/msg00112.html
https://bugzilla.gnome.org/show_bug.cgi?id=770218



Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>