Freeze break: gnome-shell: network menu overflow

[Owen Taylor <otaylor redhat com>]

- Hopefully *last* freeze break for gnome-shell I'm going to ask for
- It's conditional since I need to get another review of my
  patch, and some testing by other people.

If you have many wireless networks visible, then when you click the
More... expander on the networking menu, the list goes off the screen
and you might not be able to get to the wireless network you need.

 https://bugzilla.gnome.org/show_bug.cgi?id=646001

There's a workaround - select "Network Settings" and the network cab be
selected under the Wireless section - but it's not very discoverable.

Patch in the bug take a simple approach of adding a scrollbar to the
section when needed. I think the patch is pretty low risk because it
doesn't add any complicated tracking logic. There are a few hackish
low-level layout and event handling aspects but it's certainly not
the level of hack that there would be to add a scrollbar to a GtkMenu -
menus in shell are basically just boxes with children.

- Owen