libnm/libnm-glib: don't quit in the middle of asking for secrets (bgo #752237)

libnm/libnm-glib: don't quit in the middle of asking for secrets (bgo #752237)

If the VPN plugin terminated and the user started it again, then the
quit timer will still be running and it sometimes happens that the
VPN plugin will quit while the UI is asking the user for secrets.
That's not very nice, so don't do that.

Reproducer: while connect to the VPN, suspend your laptop.  Then
resume it, and immediately re-start the VPN connection.  Watch the
secrets dialog disappear within a very short time.

https://bugzilla.gnome.org/show_bug.cgi?id=752237