add adv. preference for number of menu items displayed at once

Not completely sure if this is necessary, but various people have expressed varying opinions about how this should work, what is most comfortable for machines with large screens vs small screens etc.

The purpose is that, we are making an effort to support having lots of add-ons active simultaneously, and then the number of wml item slots becomes a scarce resource, so we want to mitigate that and make it not a concern.

See earlier commit: f24f6adee142c088595647d5506737c48892bf38

Forum discussion: http://forums.wesnoth.org/viewtopic.php?f=6&t=40668