Using a typedef to make some code easier to read.
Turn wmi_container into a container.

This defines some types and functions that come from the STL
definition of a container. It's not complete, though, as I do
not see a reason to define size(), max_size(), or swap() at the
moment. (They would be easy to add, if the need ever arises.)

The new #include file might be useful elsewhere as well. I'll look
into that.
Make use of wmi_container's new features.
Add find() and erase() to wmi_container.
Remove wmi_container::get_menu_items().

No one needs to get at the underlying implementation anymore.
Have wmi_container::get_item() return an item instead of a pointer.

Now wmi_container is a black box, with no external code concerned
with how it stores its items.
const-qualify elements of play_controller::wml_commands_.
No need to search through a container twice.