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"
Updated default window size for Macs

Make the default window size 800 x 600 for Macs. This is a workaround for bug #20332.
Merge pull request #239 from ancestral/master

Updated default window size for Macs
fix bug 21723

https://gna.org/bugs/index.php?21723

The problem appears to be that when checking the team associated
to an item, we match it against the playing team rather than the
viewing team. This has the consequence that when it is not your
turn, you can see items that should only be visible to the
(opposing) side that is currently playing.

Tested that the bug is fixed using the following test scenario,
placed in data/multiplayer/scenarios:

    [multiplayer]
        name = "Unit Test item_visibility"
        map_data = "{test/maps/move_skip_sighted.map}"
        turns = 3
        id = item_visibility
        random_start_time = no

        {DAWN}

        [side]
            side=1
            controller=human
            name = "Alice"
            type = Elvish Archer
            id=alice
            fog=yes
            team_name=West
            share_view=no
        [/side]
        [side]
            side=2
            controller=human
            name = "Bob"
            type = Orcish Grunt
            id=bob
            fog=yes
            team_name=East
            share_view=no
        [/side]
        [side]
            side=3
            controller=human
            name = "Dave"
            type = Dwarvish Fighter
            id=dave
            fog=yes
            team_name=East
            share_view=no
        [/side]

        [event]
            name = prestart
            [item]
		x=2
		y=3
		image=scenery/rune1.png
		team_name=West
            [/item]
            [item]
		x=2
		y=5
		image=scenery/rune1.png
		team_name=East
            [/item]

            [label]
                x=3
                y=3
                text = "West team"
                team_name=West
            [/label]

            [label]
                x=3
                y=5
                text = "East team"
                team_name=East
            [/label]
        [/event]

    [/multiplayer]
Merge branch 'fixbug21723'
update changelog
update changelog
change an assertion error message associated to lua exceptions
Merge pull request #229 from cbeck88/wmi_items_preference

add adv. preference for number of menu items displayed at once,

and remaining fixups there
Merge branch 'master' of git://github.com/wesnoth/wesnoth
run test executable through gdb always, on travis

We keep getting segfaults that don't reproduce the second time it
is run, so maybe this will catch them.

It also moves the test execution code out into a script.
mark the test executor script as executable
move bom search code to a script
move ai playtest and backtrace out of travis.yml to a script