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

See earlier commit: f24f6adee142c088595647d5506737c48892bf38

Forum discussion: http://forums.wesnoth.org/viewtopic.php?f=6&t=40668
change rendering of last page of wmi's, to prevent menu resize

This commit makes the pager just show enough entries from the
previous page to ensure that we have a full page. When the number
of displayed entries is large, if we don't do this then the menu
can resize dramatically which is a bit jarring.
const-correctness fixup (just for clarity)
add [show_if]'s to the test scenario for this feature
fixup wmi_pager so that it takes account of [show_if] when paging

I also change the interface to wmi_container to be a bit more
natural.
add TODO notes about improving the wmi pager
change a label "Earlier Items" -> "Previous Items"
Merge pull request #229 from cbeck88/wmi_items_preference

add adv. preference for number of menu items displayed at once,

and remaining fixups there