Network Menu: update the UI only when really needed

Network Menu: update the UI only when really needed

By using Main.queueDeferredWork, we can ensure that most of the
menu contents (in particular, the heaviest parts like the list of
wifi networks) are not updated immediately as we receive signals
from NetworkManager. Instead, the menu is rebuilt some time later,
or as soon as the user opens the menu.
This means that it is no longer needed to optimize for the
access-point-added case, replacing a lot of buggy code with a safer
call to _queueCreateSection, which in turn should ensure that the
more menu, if existing, is always at the end and that at most 5 networks
are visible outside it.

https://bugzilla.gnome.org/show_bug.cgi?id=664124