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.
move is_observer from game_board to display_context, b/c editor uses
move a barely used function out of unit into statistics
remove pointless static singleton getter for teams
fixup map label segfault in editor
Merge branch 'fixup_editor'
remove pointless "team::nteams()" static getter
move homeless statistics calc fcns from unit.?pp to display_context
fixup syntax errors in .travis.yml
fix as_arrays for variable_access_const

previously the code
config c;
variable_access_const("a[1]", c).as_array();
would give an segfault.
now we return a range containing one empty element.
fix assertion error on lua exceptions,

lua uses c++ exceptions of type const lua_longjmp * internaly for lua
exceptions.
fix config comparison

the previous implementation would result in ["a"] == "1" beeing false if
'c["a"] = "1";' happened previously.
extend test_config unit test